ORGANIC SOLDERABILITY PRESERVATIVES:BENZOTRIAZOLES AND SUBSTITUTED BENZIMIDAZOLES

Joseph D. DeBiaseEnthone-OMI Inc.New Haven, CT

INTRODUCTIONThe acceptance and use of Organic SolderabilityPreservatives (OSPS) as replacements for Hot Air SolderLeveling (HASL) continues to grow. OSPS selectivelyprotect and maintain the solderability of a PCB’S copperfeatures (i.e. NUT pads, through-holes) by providingthermal protection against degradation during assembly.Historically one class of OSP, the benzotriazole inhibitors,has been successfully used by a number of major OEMS inselect assembly applications requiring a single heatexcursion. During the past five years a new upgraded classof OSP, the substituted benzimidazole, has proven to yieldadditional benefits to PCB fabricators while addressing adiversity of processing challenges encountered by PCBassemblers and OEMS.

Environmental and economic factors, as well asadvancements in PCB technology are the three majordrivers which have ignited the change from HASL toOSPS. Environmental concerns include the desire toeliminate hazardous materials (e.g. lead) from thefabrication production environment, as well as the need toaddress potential legislation restricting the use of lead onthe local, state, and national levels. Since OSPS areaqueous-based and do not produce hazardous waste,environmental compliance is made easier.

Economically, the OSP process is substantially lessexpensive than HASL, which is a labor intensive and ahigh maintenance process. When one considers that theHASL process produces solder dross (considered ahazardous material), the disposal of the dross is both aneconomic and environmental burden that the PCBfabricator would welcome the opportunity to eliminate.When one compares actual processing cost, reworkhepaircost, and the cost of rejects, current users of substitutedbenzimidazole-based OSPS have experienced a total costsavings of 25 to 50°/0 versus HASL. For benzotriazole-based coatings the savings are even more impressive.

Finally, technology advancements evidenced by the adventof new and complex packaging designs such as fine-pitch,ball grid arrays (BGAs), chip scale, and flip chip havenecessitated flat, planar surfaces. Another processingconcern is the trend towards thinner pCBS.

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HASL has limited success in processing PCMCIA cardsand thin PCBS. It is not uncommon for HASL to produceboards with inconsistent solder coating thicknesses, thuscausing “crowned” SMT attachment areas and reducedclearance of through-holes. These deficiencies result insubsequent rework and rejects at assembly and reduce first-pass assembly yields. Conversely, OSPS are known toconsistently produce thin coatings. Other problemsassociated with the HASL process include: intermetalliccompounds formation (Cu/Sn) and the potential for poorsolderabilty; warpage due to changes in temperature; andthermal shock which jeopardizes PCB integrity whileescalating the potential for increased ionic contaminationlevels.

This article will examine the considerations required inselecting benzotriazole and substituted-benzimidazoleOSPS for implementation in PCB manufacturing.

OSPS IN PCB FABRICATIONBy implementing OSP technology the PCB fabricatorreplaces a hazardous, difficult to control process (HASL)with a lead-free, easy to control, aqueous process. Figures 1and 2 outline typical benzotriazole and substitutedbenzimidazole process cycles.

The front end of each process, referred to as the “pre-cleancycle”, is quite similar. During this cycle the PCB is firstimmersed in a cleaner (typically an acid cleaner) to removeany finger oils or other contaminants. Some copper oxideremoval also occurs. During the microetching step thecopper oxide is fully removed and a total of 20-40microinches of copper is etched away. The resultant coppertopography has an increased surface area due to themicroroughened surface. This surface results in anincreased number of attachmenthonding sites for the OSP.Additionally, the associated high sutiace energy of thecopper will result in a high solderability level. The result ofpoor or insufficient microetching is a non-uniform, lessthan ideal coating appearance. This may also negativelyallect solderability during wave soldering. The acid rinsestep is used to neutralize any microetchant salts not fullyrinsed off, as well as a sacrificial bath to inhibit anycontaminants from entering the OSP solution. A 5-10°/0sulfhric acid (H2S04)solution is typically employed for thisoperation.

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After the copper is properly prepared, OSP application canoccur. The benzotriazole coating is deposited by immersingthe PCB in a 1.0 to 2.5% concentration solution for 60~30seconds. The solution temperatures can range from roomtemperature to 49”C. A heated solution will aid in dryingthe PCB. The most critical coating control parameter ismaintaining the solution pH above 3.0. The resultantdeposit is a monolayer approximately 50 to 150 Angstromsthick. A thicker deposit is not possible. See Figure 3 for abenzotriazole bonding mechanism onto copper.

The substituted benzimidazole coating is deposited byimmersing the PCB in a 100 ~ 10°/0concentration solutionfor 45 to 90 seconds. The solution temperature is set at-43°C and the pH at -2.7. The resulting coating thicknessis 0.35 ~ 0.15 microns (3500 ~ 1500 ‘A’). The protectivelayer is formed initially through a completing reactionwith copper to form an organometallic bond, followed by abuild-up of the benzimidazole to the requisite finalthickness. The deposition process is controlled via thecoating thickness. The solution temperature and dwell timeare kept constant during operation. The solutionconcentration and pH/total acidity will change over time,and thus are monitored and replenished accordingly. SeeFigure 4 for a substituted benzimidazole bondingmechanism onto copper.

The $inal step in the OSP application process is PCBdrying. Drying should be performed in an in-line dryer andshould be of sufficient temperature (60-80”C), air reflowand duration to ensure that all PCB holes and vias are freeof water. This step is critical for achieving maximum shelflife and solderability of the OSP-coated PCBS.

These OSP coating processes can be pefiormed either in avertical dip line or in an automated horizontal line. Formaximum consistency and throughput, especially of smallPCBS or PCBS containing small holes @ 0.015”), ahorizontal process is recommended. Horizontal processingequipment for both types of OSPS is quite similar. The onlymajor difference is that benzotriazoles can be applied viaspraytechnology, whereas substituted benzimidazoles, dueto their long molecular structures, can not. Benzimidazolesare successfully being deposited in production via ahorizontal flood immersion module being used in the OSPcoating step. See Figure 5 for an example of a typicalhorizontal immersion processing line.

Unlike HASL, and other metallic PCB surface finishes,OSP-coated PCBS are reworkable by simply re-processingthem through the original application process. If necessa~,rework of the OSP coatings can occur several timeswithout concern for the integrity of the PCB. The onlycritical parameter to monitor is the total amount of copperremoved during the microetch steps. Once a total of 60 to80# inches has been removed it is best to skip themicroetch step if subsequent reworks are required. This

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should suflice since the copper PCB surface topographyoriginally created will remain intact over time.

The recommended procedures for packaging OSP-coatedPCBS for shipping to the assembler are quite similar tothose for HASL. The PCBS should be stacked 10 to 25high, depending on size, without individual slip sheetsbetween each PCB. An outer slip sheet, made from sulfbr-free materials, should be placed on the top and bottomPCBS in the stack. A “shrink wrap” should be applied totightly seal the PCB stack. The PCB stacks should beplaced in a box which includes desiccant. During thepackaging step operators should wear impermeable glovesin order to eliminate the possibility of ionic contaminationand coating degradation (especially for benzotriazole -coated PCBS).

RELIABILITY OF THE OSP COATINGSThe reliability of OSP coatings can be defined by howdurable they are prior to assembly (i.e. shelf life, handling,robustness, etc.) and by their long term electrical insulativeproperties.

DurabilityHistorically one of the main concerns with the use of OSPShas been their limited shelf life and low tolerance to beinghandled. The benzotriazole coating is a thin, monolayerdeposit that is quite fragile. This coating is susceptible tohigh humidity conditions and the best case storage/shelflife is typically 3 to 6 months. Finger prints or othersimilar handling soils will adversely affixt thebenzotriazole coatings. Furthermore, the coating’sprotective properties are severely diminished when exposedto temperatures exceeding 100”C. After one soldering heatexcursion (e.g. SMT reflow) this coating rapidly degradesand severe oxidation of the emaining copper stiacesfollows, thus rendering assembly of the PCB’S second sideimprobable.

Over 20 years of industry production experience has welldocumented the limitations of the benzotriazole coatings.Despite these limitations, numerous OEMs/assemblershave successfully used these coatings as HASL alternativesby optimizing their assembly processes to compensate forthese shortcomings. These coatings have been most widelyused in the computer and telecommunications sectors inconjunction with single soldering assembly processes,highly active soldering materials and/or inerted solderingprocesses.

Through six years of worldwide production experience thesubstituted benzimidazoles have been demonstrated to besubstantially more robust than previous OSPS. Acceleratedaging, along with actual production experience, has foundthat these coatings maintain solderability for a minimum oftwelve months, at typical assembly shop conditions (e.g. s35°C, < 80°A R.H.). 1 Companies that have pefiormed

accelerating type tests have learned not to use steam aging,but rather more moderate conditions.

For many years, steam aging has been used to artificiallyaccelerate the storageishelf life conditions of solder coatedPCBS. The steam conditions (90-96”C) were ideal forinducing the growth of the non-solderable Cu-Snintermetallic compounds (IMCS) found on PCBS coatedwith solder. In a relatively short time (8 -24 hours) a shelfltie period of more than one year could be simulated.Companies looking to use this method on OSPS have foundthat it is not suitable. Since IMCS are not present in a OSPcoated PCB, the steam aging only serves to degrade theorganic coating and oxidize the copper. Therefore othermeans of accelerated testing are necessitated. Testing hasbeen reported with 35”C, 95’XOR.H. for 96 days, 65”C, 95’%0R.H. for 24 hours and 40”C, 90% R.H. for 1000 hours. 2’3These conditions all simulated 12 to 24 months of storagelife.

Although metallic PCB surface finishes tend to be morerobust to handling then OSPS, the substitutedbenzimidazole coatings are significantly more tolerant thanthe benzotriazoles. Production experience at numerouscontract assemblers in the U.S. has shown that thesecoatings can withstand the routine handling that PCBS aresubjected to during assembly without the need for gloves.Of course proper handling of PCBS by their edges shouldalways be stressed, regardless of PCB surface finish, inorder to minimize the potential for ionic contamination.This is most important for PCBS to be used in no-cleanassembly processes.

Long Term Electrical ReliabilityTo date, numerous reliability tests have been performed tocharacterize the effect of the presence of the substitutedbenzimidazole coating on the long term electricalproperties of the PCB. These tests were performed todetermine the reliability of the coating “as deposited” aswell as after one or more heat excursions (solderingoperations). Following is a summary of some of these tests:

. JIS Z-3197 Surface Insul&”on Resistance

JIS Z-3197 Type 2, interlocking comb patterns with 12.5mil lines/spaces and a 0.62” overlap length of the fingerswere coated with a substituted benzimidazole coating andsubjected to 1000 hours at 40”C, 90°/0 R.H. Insulationresistance measurements were taken at 100 VDC testvoltage every 200 hours. After 1000 hours the finalinsulation resistance was 2 x 1013ohms. See Table 1 foractual readings.4

. Bellcore TR-NWT-000078 Surface Insulation

ResistanceIPC-B-25 comb patterns with 25 mil lines/50 mil spaceswere coated with a substituted benzimidazole coating and

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subjected to 96 hours at 35”C, 85-90’XOR.H. and a 45-50VDC bias. Insulation resistance measurements were takenevery 24 hours at a 100 VDC test voltage. After 96 hoursthe final insulation resistance was >3 x 1012ohms (Pass=1.2 x 10” ohms). See Table 2 for actual readings.

The same test was petiorrned but after the test couponswere coated they were subjected to one SMT reflow in anIll/convection oven in an air atmosphere. The finalinsulation resistance for the “soldered” coupons was >7 x

1012ohms (Pass = 1.2 x 1011ohms). 5

. Modljied Bellcore TR-NWT-000078 SurfaceInsul&”on Resistance

IPC-B-25 comb patterns with 12.5 roils lines/spaces werecoated with a substituted benzimidazole coating and weresubjected to various reflowlwave soldering conditions priorto electrical testing. The SIR test conditions were 35”C, 85-90’XOR.H. 45-50 VDC bias for 28 days. Insulationresistance measurements were taken at 100 VDC testvoltage every two days. The minimum acceptableinsulation resistance is 2 x 1010ohms.

Condition 1 : Coating subjected to one SMT reflow.Final I.R. was> 3 x 1012ohms.

Condition 2: Coating subjected to five SMT reflows.Final I.R. was> 4 x 1012ohms.

Condition 3: Coating subjected to one WIT reflowplus wave soldered with a low solids,no-clean flux

Final I.R. was> 3 x 10” ohms.

Condition 4: Coating subjected to five SMT reflowsplus wave soldered with slow solids, no-clean flux.

Final I.R. was> 3 x 1011ohms.

See Table 3 for actual readings.

Through the use of numerous SMT reflows it was proventhat the coating is highly inert even after expsure toelevated temperature excursions. The introduction of thelow solids flux demonstrated the suitability of the no-cleansystem, which incorporates the use of the substitutedbenzimidazole coating in a no-clean, mixed technologyassembly.

Furthermore, Condition 4 simulates the worst case scenarioof having exposedhon-wetted copper present with tin-leadalong with no-clean flux residues, which could occur whenfhll wetting is not achieved during wave soldering. The testresults prove this to be an inert system and probably mostdependent on the activity of the no-clean flux used.

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● Bellcore TR-NWT-000078 Electromigration

ResistanceIPC-B-25 comb patterns with 25 rnil lines/50 mil spaceswere coated with a substituted benzimidazole coating. Halfof the coupons were subjected to one SMT reflow. All ofthe samples were stabilized at 85°C, 85V0 R.H. for 96hours. Initial insulation resistance measurements weremade using 100 VDC test voltage. The coupons were thensubjeeted to 500 hours at the 85°C, 85 R.H. and 10 VDCbias conditions. The final results showed no decrease in theinsulation resistance as a result of the applied bias.Additionally, at 10X magnification there was no evidenceof electromigration or dendritic growth. G

Q!!!PQ! Initial LR Final LRControl 6.4 x 108Q 3.6 X 108QAs Coated 3.8 X 108f2 1.2 X109QReflowed 7.2 X 108Q 1.1 X109Q

. Mod&iedBellcore TR-NW-000078Electromigr&”on Resistance

IPC-B-25 comb patterns A, B and C were coated with asubstituted benzimidazole coating and subjected 85°C, 85°/0R.H. for 1000 hours at three different applied voltages.Each group of coupons had their own uncoated coppercontrol comb patterns.

AEll!NTest Board Combs Voltaze BiasPattern A 6 mil lineskpaces 50 VDcPattern B 12.5 mil lines/spaces 100VDcPattern C 25 mil lines/50 rnil spaces 500 VDc

Initial insulation resistance for all comb patterns, includingcontrols, were -1 x 1014ohms. Mer 1000 hours all thecoated coupons were measured at >5 x 109 ohms, with thecontrol coupons all within less than a decade difference. At10X magnification no evidence of any electromigration ordendritic growth was reported. See Table 4 for actualreadings.’

. Moisture and Insul&”on Resistance -MIL-STD-202E, Method 106D

Per MIL-STD-202E IPC-B-25 comb patterns wereprepared with a liquid photoimageable solder mask. Halfwere coated with a substituted benzimidazole and theremainder with HASL. The coupons were first cycled from25°C, 90V0R.H. to 65°C, 95’%0R.H. for ten 16 hour cycles.Once pre-eonditioned the comb patterns were measured forinitial insulation resistance and then subjected to 65°C,95’XOR.H. for 139 hours with a 100 VDC bias. Thespecifkation requirement is that the final insulationresistance must be >5 x 108ohms.

m Initial I.R Final I.RHASL 2.0 x 10’2Q 6.0 X 107G?OSP 1.3 x 10’3Q 4.0 x 10’1Q

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No failures were attributed to solder mask degradation.The failure analysis did suggest that the presence of thesolder mask was responsible for entrapping someconstituents from the HASL flux during the hightemperature coating operation. As the solder mask softenedat the elevated temperature, cycling and bias of the test,these potentially ionic materials were released.

Although the benzotriazole coatings have been used formany years, the volume of published reliability data is notsubstantial. Most large users have determined the coatingto be reliable via similar tests to the ones deseribed above,but have never released the results into the public domain.See Table 2 for the results of one Bellcore SIR test.

OSPS IN PCB ASSEMBLYThe SMT ProcessThe major benefit of OSPS to the assembler is the flat PCBsoldering pads. Unlike HASL-coated PCBS there is nosolder bump on these pads to interfere in solder pastedeposition and component placement. The flat surfaceallows the assembler maximum control in depositing anaccurate, consistent volume of solder paste. This results ina higher transfer efllciency of the solder paste especiallyfor stencils with small apertures (i.e. FPT, UFPT, BGA,micro-BGA, flip chip). This “cleaner” solder paste depositsignificantly reduces the incidence of lateral smear of thesolder paste on the underside of the stencil and of solderpaste bridging of tight pitch soldering pads. With finerpitches (< 0.020”) this will minimize the need to reducestencil apertures, which will aid printability, withoutconcern for bridging. If minimizing the amount ofexposedhon-wetted copper on the periphery of an SMTpad is important, then the stencil aperture/SMT pad ratioshould be maintained at 1:1. The recommended maximumreduction for FPT apertures (in air reflow) is 20°/0 (i.e. 8mil wide paste deposit on a 10 mil wide pad). Aperturereductions in the lengthwise direction are unnecessary andnot recommended.

Due to the high color contrast of the materials in use, (e.g.OSP-coated copper, the solder mask and solder paste),additional in-process benefits are realized. Alignment ofthe stencil to the PCB during screen printing set-up isfacilitated due to the copper color of the SMT pads. Anymissed or skipped solder paste deposits onto the PCB willbe easily detected because of the copper pad to solder pastecolor contrast. This will allow earlier deteetion of poorprinting and will preclude these PCBS from being fillyassembled, only to require subsequent rework. Finally, theflat non-reflective topology of OSP<oated fiducial marksallows more consistent recognition by the assemblyequipment’s machine vision.

Production experience to date has exhibited excellentcompatibility of most solder pastes, regardless of the flux

vehicle activity (i.e. OA, RMA, no-clean, low residue) withthe benzotriazole and substituted benzimidazole coatings.In converting to the OSP processes there is no need toadjust the thickness of the solder paste deposited. Normallythe same thickness used with HASL PCBS will suflice.Most of the solder from the HASL process remains underthe NUT leads during reflow and does not contribute to thesolder fillets formation.

Existing SMT reflow profiles used for HASL - coatedPCBS should work well for both OSP types, Although,some optimization to the profile will enhance second sidesolderability. Maintaining the peak temperature that thePCB experiences (in air) during first side reflow below240”C will be beneficial for subsequent solderingoperations. Similarly, increasing the post-reflow PCB cool-down rate to reach 100”C as quickly as possible is alsoadvantageous (not to exceed cooldown rate specified forceramic components). These optirnizations will increasethe process window for substituted benzimidazole coatingsfor multiple heat excursion assembly. Utiortunately thisbeneficial effect does not extend to the benzotriazolecoatings. After one heat excursion the second side copperis considerably oxidized. Consistent second side solderingis unlikely, even with highly active soldering materials.

One consideration that is required during SMT assembly ofsubstituted benzimidazole coated PCBS is the selection ofsolvents used to wipe down misprinted solder paste. Thesecoatings are soluble to varying degrees in most solvents(acidic or basic). Even though some amount of coating maybe removed by a chosen solvent, this does not equate to aloss of solderability. Careiid selection of an appropriatewipedown and cleaning solvent will eliminate potentialincompatibilities. The benzotriazole coating, as a result ofits strong monolayer bond to copper, is not similarlytiected.

In most in-line assembly processes all of the solderingsteps will occur within a few hours (if not minutes) of eachother. At times, due to equipment breakdown orcomponent shortages, partially assembled PCBS may incursubstantial hold times prior to subsequent soldering.Typically, for substituted benzimidazole coated PCBS ifthese delays are a matter of hours or even days,solderability of the second side will not be compromised.Several variables will impact the maximum hold timeallowable:1. Time at elevated temperature during the first solder

paste reflow.2. Maximum temperature the PCB reached during the

first reflow.3. Reflow environment used (N2vs. Air).4. Cleaning of the PCB and what type of solvent used.5. Solder paste wipe down for misprint.6. Activity of soldering material to be used for side tsvo.

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Production experience has shown that hold times of one toseven days are typical and should not be of a concern. Thevariables listed above will dictate the acceptable hold timesfor PCBS in a particular process. One major OEM hasexperienced hold times greater than one month withoutsolderability degradation.8

The Wave Soldering ProcessProduction experience to date of single pass through-holewave soldering of benzotriazole and substitutedbenzimidazole coatings has exhibited excellentcompatibility of most flux types, regardless of activity (i.e.OA, RA, RMA, SA, low solids no-clean, VOC-free noclean) or flux application technique. For mixed technologyassembly, where wave soldering occurs after prior heatexcursions, the more active flux types will be fidlycompatible with the substituted benzimidazole coatings.Low activity soldering materials such as low solids andVOC-free no-clean fluxes may require some processoptimization. Benzotriazole coatings are not recommendedfor use in mixed technology processes.

Successfid implementation of low activity fluxes requiressome scrutiny of the wave soldering process. Typically themajor areas to be analyzed are the flux formulation, theeffectiveness of the flux application method/equipment andthe assembly process parameters. The choice of flux andhow it is applied to the PCB’S plated through-holes (PTHs)are major factors in determining the level of hole fill andtop side wetting achieved. The wetting mechanism forOSPS differs from HASL. In order to achieve good wettingthe flux needs to be applied where the solder needs to flow.The flux is instrumental in penetrating the coating aheadof the molten solder. The more complete the fluxpenetration into the PTHs, the more likely that fill hole filland top side wetting of the annular ring will occur. Fluxapplication techniques like wave fluxing and foam fluxingare effective at delivering a suitable level of flux transfer tothe PTHs. These techniques result in excellent hole fill andtop side wetting even with the lowest activity fluxes.Conversely, most spray fluxing equipment is limited in itsability to deliver flux far enough up into the PTHs.Additionally, spray fluxing significantly reduces thequantity of flux applied to the PCBS. For these reasonsunoptimized spray processes will typically yield poor topside solder fillets.

Low solids and VOC-free no-clean fluxes specificallyformulated for OSPS are now available from numerous fluxmanufacturers. These fluxes typically offset some of theseflux application shortcomings by being slightly moreactive, having superior capillary action or being moreeffective at penetrating the OSP. A new flux applicationtechnique, the JetFlux@ System, uses the principles of inkjet printing to achieve superior flux transfer to the PTHs.The flux is delivered in liquid micro streams that depositwell up into the PTHs to be soldered. This technique helps

achieve excellent top side fillets while applying a fluxquantity similar to spray fluxing.

In converting to mixed technology assembly of substitutedbenzimidazole coated PCBS with low solids or VOC-freeno-clean fluxes, several process parameters should bereviewed and optimized:

1. Improve PCB first side SMT reflow process control byminimizing the peak reflow temperature andincreasing the PCB cooldown rate.

2. Minimize PCB hold times between solderingoperations.

3. Maintain wave fluxing PCB top side pre-heattemperature at 100-12O”C.

4. Maintain solder pot temperature at 245 to 260”C.5. Use air knife after flux applicator to assist flux

penetration up the PTHs.6. Use turbulent type wave cordlguration (e.g. chip wave)

to maximize hole fill.7. Increase wave contact time and PCB depth in solder to

maximize hole fill.

Optimization of the above listed common wave solderingparameters, along with selection of the appropriate fluxand application technique will result in successful no-clean, mixed technology assembly with OSPS. See Table 5for a summary of OSP compatibility with various assemblyprocess and soldering materials.

Effect of Nitrogen InertingHistorically, nitrogen inerting of SMT reflow has beenused to maximize first-pass assembly yields for processesusing low activity soldering (no-clean) solder pastes. Mostrecently nitrogen, in the form of a “blanket” surroundingthe solder pot, has been used in wave soldering with lowsolids, no-clean fluxes. In this case the benefits stem morefrom the reduction of dross formation on the solder potthan increased solderability. In the case of OSPS nitrogeninerting of the entire assembly process is beneficial.

Some assemblers have found that by inerting the SMTreflow process they are able to “extend the effectiveness ofbenzotriazoles for some multi-heat excursion processes.When inerting is coupled with the use of highly aggressivewater soluble wave solder fluxes and short hold timesbetween soldering operations, some success with mixedtechnology assembly may occur. Only a few companieshave ever succeeded in consistently achieving this.

The use of nitrogen (< 500 ppm Oz)in the SMT reflow ofsubstituted benzimidazole coated PCBS will improve thewave soldering hole fill with low activity fluxes. DuringSMT reflow of the top side of the PCB, the bottom sidealso experiences the elevated temperature and thus thecoating on this side also “opens its pores”, but does notcome in contact with the flux vehicle. During this period

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some oxygen diffusion will occur through the coating. Theuse of a nitrogen atmosphere will reduce this condition.When the PCB temperature is reduced after reflow, thecoating will “close its pores”, and thus not allow anyfurther degradation. By reducing the quantity of oxygendiffhsed through the coating on PCB side 2 during SMTreflow, the second side soldering operation will experiencea wider operating range. For this reason, OSPS retainsolderability through more heat excursions in nitrogenversus air. When nitrogen is used in wave soldering, butnot during SMT reflow, one will experience some increasein process latitude. This will not be as significant animprovement as when nitrogen is used during SMT reflowalone. OSP-coated PCBS soldered in air will remaineffective through 3 to 5 mass heat excursions (dependingon the activity of the soldering materials) plus anylocalized soldering (i.e. hand soldering, rework, etc.). Innitrogen (< 500 ppm 02), more than 5 mass heat excursionsare possible.

Production experience has shown that the use of nitrogen isnot required for successfid mixed technology assembly ofOSP-coated (substituted benzimidazole) PCBS in a no-clean environment. The key is proper process optimization.

APPLICABILITY TO NEWPACKAGING TECHNOLOGIES

In general both types of OSPS will be beneficial to anypackaging technology where soldering to a flat attachmentarea is required. The following is a summary ofapplicability to various new technologies.

Ultra Fine-PitchUltra fine-pitch is defined as lead-to-lead spacing of 0.016”or finer. Work to date has shown that the pad flatnessafforded by the OSPS, in conjunction with finer meshsolder paste powder (-400/+500, -500/+635) makesproduction level processes fmsible below 0.015” pitch.With the recent advances in stencil manu-facturing andscreen printer accuracyhepeatability, pitches down to0.010” may be feasible.

Ball Grid Arrays (BGAs)To date a substantial amount of production work has beenpetiormed with various BGA configurations soldered toOSP-coated PCBS. A recent study by a major OEM foundthat the use of certain types of substituted benzimidazoleOSPS resulted in the highest solder joint reliability forBGAs. This test compared HASL, various Ni-Au coatings,Sn-Bi, Ni-Pd, and various OSPS.

The flat pad attachment areas offer the potential for everyBGA solder joint to be virtually identical in solder volume.By improving the probability of forming consistent solderjoints the opportunities for costly rework are virtuallyeliminated. For ideal compatibility with OSPS, the BGAsneed to have eutectic solder balls which will be attached to

the PCB via fluxholder paste or high temperature alloysolder balls which will use solder paste for attachment.Micro-BGAs, mini-BGAs, and other BGA configurationswill all benefit similarly.

Flip ChipFlip chips, due to their low profile solder bumps, will alsobenefit from consistent, flat pads. Flat pads will tolerateany lack in bump coplanarity better than a poorlycontrolled HASL deposit. In a similar manner to BGAs,any flip chip configuration where the bump is made of analloy that will melt during standard reflow will be fillycompatible with OSPS.

Tape-Automated-Bonding (TAB)Soldering of TAB components normally requires that thesolder be pre-deposited on the attachment pads (which aretypically 0.010” pitch or finer). OSPS are not directlycompatible with TAB attachment but they may be of use bycreating a flat solderable surface for applying the pre-deposited solder. The solid solder techniques often used inthis application would be compatible with the OSPS.

Chip-on-Board Wire BondingThe OSPS will not be directly compatible with any type ofwire bonding. OSPS maybe usefid for PCBS requiring bothwire bonding and solderability. Wire bonding needs tooccur to a noble metal like gold, but often the goldthickness employed could be detrimental to the formationof metallurgically sound solder joints. Therefore OSPS canbe used for PCBS requiring two surface finishes. Once thePCBS have been gold plated in the wire bond attachmentareas they can be processed through the OSP coatingoperation. Unlike HASL and other metallic finishes,masking of the gold is not necessary.

.PCMCIA Cards - Thin CardsMost PC card designs use a 0.016 to 0.018” thick PCBwith numerous fine pitch components. These PCBS aretypically fabricated in 18’ x 24” panels. It is very difIlcultto HASL such large, thin panels consistently. The use ofOSPS on such designs results in increased first-pass yieldsof the fine-pitch components by reducing the incidence ofbridging. Additionally the mild temperatures of the OSPprocess will not alter the dimensional integrity of the thinC Cards. The thermal shock associated with the HASLprocess can distort the physical dimension of the PC cardsin a non-linear manner. This distortion, albeit relativelysmall (.1 to 2 roils), can be enough to affect the screenprinter and component placement accuracy, thus impactingfine pitch assembly yields.

SOLDER JOINT RELIABILITYOne of the primary issues in considering new PCB surfacefinishes is the reliability of the resulting solder joints.Numerous reliability tests have been performed byautomotive, computer, and telecommunication OEMS.

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Several of these tests comparing substituted benzimidazoleOSPS to metallic surface finishes are summarized below.

Pull Strength TestNumerous companies have performed comparative pulltests for solder joints formed OSP and HASL-coated PCBS.A major U.S. computer manufacturer performed thefollowing test:

PCB preconditioning: Thermal cycling from-lo” C to110”C, 30°C/rninute ramprate, 1 minute dwell atextremes for 4000 cycles.

Pull and peel tests were performed on 7000 solder joints.All recorded strengths for the OSP formed solder jointswere equal to or greater than for the HASL ones.

Vibration TestThe same computer manufacturer fabricated test vehiclesthat incorporated daisy chained devices. The test vehicleswere subjected to a vibration stress range of 10 to 2000hertz on 3 axes at 12G’s for 20 hours. No opens werefound for either the OSP or HASL-formed solder joints.g

Bend Flex Test (3 Point)A major telecommunications company petiormed thefollowing test to compare HASL, Ni-Au and OSP-coatedPCBS.

Test Vehicle;

Pre-Conditioning:

Test:

A 41 I/O ceramic BGA (0.52” x0.47”) with 0.30” solid spheres(Sn62/Pb36/Ag2) was reflowedonto an 0.045” thick FR-4 PCB.

The assembled PCBS were thansubjected to an additional SMT’reflow to simulate second sideassembly. The PCBSwere then artificially aged for 24hours at 100”C.

A known load (force) wasplaced on the PCBS andincreased until a fracture of thejoints was detected.

Results:Load (lbs)

.,TyPq ,, .,Mean ~ta~dat~ ,~yiatio+HASL 87 11OSP 107 6

NifAu 62 24

The solder joints formed from OSP coated NUT pads weresignificantly stronger than those joints formed with HASL

or Ni/Au. The standard deviation results showed that OSPformed solder joints were also most consistent. DuringHASL, a thick, non-uniform Cu-Sn intermetalliccompound (IMC) layer is formed at the copper-solderinterface, an area prone to solder joint fracture. In the caseof Ni/Au, the ftilure occurred at the copper-nickelinterface. The nickel deposit exhibited poor ductility andwas the weak point in the solder joint. Solder joints formedfrom OSP-coated pads have seen only one thermal cycle(that during reflow). This cycle is well controlled in timeand temperature and thus allows formation of IMCS with afine, homogeneous crystalline structure. This leads tostronger, more ductile solder joints.

Thermal Shock TestThe same telecommunications company performed athermal shock test comparing HASL, various Ni-Aufinishes, Sn-Bi, Ni-Pd and two OSPS.

Test Vehicle; Same BGA vehicle as used in the3 Point Bend Flex Test.

Pre-Conditioning; Same

Test: PCBS were subjected to a liquid toliquid thermal shock from -55 to125°C with a 5 inute dwell atextremes.

At 250 cycles 3 of the Ni-Au finishes and the Ni-Pd all hadexperienced some level ofjoint cracking. Anywhere from 4to 12’XOof these solder joints had experienced cracks whichcovered 50°/0 of the joint area. They were removed fromfurther cycling. At the end of the test (450 cycles) thefourth Ni-Au finish exhibited 30% of the joints as cracked,Sn-Bi had 12Y0,one of the OSPS had 13%, HASL had 6%and the second OSP had none. This OSP was the same onethat excelled in the 3 point bend flex test.

All reliability testing performed to date has exhibitedsubstituted benzimidazole OSPS as being equal to or better,in solder joint strength and consistency than HASL andother metallic surface finishes. 10

OSPS IN PRODUCTIONBenzotriazolesBenzotriazoles have been successfully used as a HASL-altemative for nearly 25 years by the computer,telecommunications and consumer electronics industry.Products as diverse as laptop computers, telephones andtelevision remote controls are manufactured with PCBScoated with benzotriazoles. The manufacturing limitationwith benzotriazole coated PCBS is that the design mustinvolve only one heat excursion or that soldering bepetiormed in an inert atmosphere with high activity fluxesfor double sided or mixed technology designs.

770

Substituted BenzimidazolesSubstituted benzimidazoles have been successfully used inall major sectors of the electronics industry including:computers, telecommunications, automotive, consumerelectronics, instrumentation and business equipment. Someof the high volume manufacturing processes currentlyusing these OSPS include:

●

●

●

●

●

●

●

A major instrumentation OEM has been assemblingnetwork display monitor PCBS for 2 years in a mixedtechnology process using a no-clean solder paste and alow solids, no-clean flux in an air atmosphere. Theflux is applied via a spray fluxer.A major manufacturer of industrial controls forautomation is assembling PCBS in a mixed technologyprocess using no-clean solder paste and a VOC-free,no-clean flux in an air atmosphere. The flux is appliedvia foam and wave fluxers.A major contract assembler has been manufacturinglap top computers for 4 years in a double sided SMTprocess using water soluble solder pastes in an airatmosphere.A major contract assembler has been manufacturingdesk top computers for 2 years in a mixed technologyprocess using a no-clean solder paste and a VOC-free,no-clean flux in an air environment. The flux isapplied via foaming.A major automotive electronics OEM ismantiacturing control modules in a mixed technologyprocess using a no-clean solder paste and a low solids,no-clean flux. The flux is applied via wave fluxing andthe solder pot uses a nitrogen blanket.A major telecommunication OEM has beenmanufacturing personal pagers for 4 years in a doublesided SMT process using no-clean solder paste in anitrogen atmosphere.A major computer disk drive manufacturer has beenusing OSPS for 2 years in a mixed technology processusing a no-clean solder paste and a low solids no-cleanflux in an air atmosphere. The flux is applied via foamor spray.

SUMMARYOrganic coatings such as benzotriazoles and substitutedbenzimidazoles have been successfdy used for a numberof years in the electronics industry. They have proven to benot only reliable alternatives to HASL but down rightessential for achieving acceptable first-pass yields forcertain complex packaging technologies, Thebenzotriazoles will probably always have a niche in theindustxy. Meanwhile many satisfied users predict that thebest of the substituted benzimidazoles will be the highvolume, low cost coating alternative to HASL for years tocome.

REFERENCES1. Banerji, K. And Bradley, E. “Manufacturability and

Reliability of Products Assembled with New PCBFinishes,” Proceedings Surface Mount International,1994, pp. 584-595

2. Short, B. and Reilly, D., “Switching From Tin/Lead toCopper Finishes,” Proceedings Nepcon West, 1993,pp. 405-415

3. Parker, J.L. and Horton, J.S., “Assembly Pefiormanceof Printed Wiring Boards Coated with an OrganicSolderability Preservative,” Proceedings SurfaceMount International, 1992, pp. 499-513

4. Tamura Kaken Internal Report, 1991

771

5. Trace Laboratories Test Report for Enthone-OMI,October 1993

6. Ibid 57. Tamura Kaken Internal Report, 19928. Platt, S. and Brantingham, J., “Determination of

Product Characteristics and Process Configuration forOrganic Solderability Preservatives,” 1994, pp. P13-4-1 through P13-4-8

9. Ibid 210. Ibid 1

@ JetFlux is a registered trademark of PrecisionDispensing Equipment, Inc., Bay Village, Ohio

—— —

Insulation Resistance (Q)

TestBoard Initial 200 Hours 400 Hours 600 Hours 800 Hours 1000 Hours

OSP IX10’4 4X10’3 4X10’3 3X10’3 2X10’3 2X10’3

Control IX10’4 6X10’3 6X10’3 5X10’3 5X10’3 4X10’3

TABLE 1. Surface Insulation Resistance (SIR) - JIS Z 3197

Average Log (Q)

+ Ban Copper control

4 -a- Benzotriazole

m -A- substituted

12- Benzimidazole

11 I I I I I I 1 I I I

o

TABLE 2.

24

Surface

6 8 10 12 14 16 18

Days

Insulation Resistance (SIR) - Bellcore

772

. ..

Average

14

13

12

11

10

9

Log (K2)

I

I

A

4

1

1 1 I I I I I I [ I I I I I I I 1

+ 5 Reflow+ I Reflow

+ 5 Reflow w/LsF

+ I Reflow w/LsF

+ copper control

-m-AS coated

O 4 8 12 16 20 24 28

Days

TABLE 3. Surface Insulation Resistance (SIR)Bellcore With Low Solids, No-Clean Flux

Insulation Resistance (Q)

Test Board Initial 250 Hours 500 Hours 1000 Hours

OSP Pattern A * 7.5 x 10’3 I.oxlo’” 7.5 x 109 5.3 x 109

Control A 8.1x 10’3 5.0 x 109 3.8 X 109 2.9 X 109

OSP Pattern B* 9.3 x 10’3 1.2x lo’0 I.lx lo’” 9.2 X 109

Control B 1.2 X10’4 1.3x lo’0 I.ox lo’” 9.3 x 109

OSP Pattern C* 2.7 X 10’4 lmoxlo’o 8.2 X 109 6.0 X 10g

Control C 2.4 X 10’4 I.oxlo’” 8.9 X 109 6.7 X 109

* No evidence of electromigration

TABLE 4. Electromigration Resistance - Various Voltages

773

Assembly SubstitutedType Benzotriazoles Benzimidazoles

Single-sided SMT

Rosin E ENo-Clean E EWater-Soluble E E

SingleSided Wave

Rosin E ENo-Clean G EWater-soluble E E

Double-Sided SMT

Rosin P ENo-Clean P EWater-soluble F E

Mixed Technology

Rosin P ENo-Clean P GWater-Soluble F E

TABLE 5. Organic Coatings - Assembly Compatibility

HZOAcid -bClean Rinse

3040”C3045 sec.

HZOMicro ~

Etch Rinse

HZOAcid OSP

Rinse Rinse Coat

25-30°C !%lo!io H2SOd 40-45°C30-4s Sec20-40 pinchremoval

20-25°C 45-90 sec.

15-30 sec. 90-1109’0

90-120?40

-L-DI

DryRinse

60-60”C

P = PoorF = FairG = GoodE=

Concentration

0.24J.5 Microns Thick

FIGURE 1. Typical Substituted Benzimidazole Application Process

774

AcidClean

-i

HZOMicro

Rinse Etch

25-30”C30-40”C3045 sec. 3045 sec

2040 pinchremoval

H20

Rinse

HZOAcid OSP

Rinse Rinse Coat

I I I

5-10% H#Od 20-49°C20-25°C 30-90 sec.

-ElDI

DryRinse

60”C

15-30 sec. pH >3.0

1.O-2.5?40Concentration

50-150 ~ Thick

FIGURE 2. Typical Benzotriazole Application Process

H H

●

H

●

H

●

Cu Cu Cu Cu Cu Cu Cu Cu

FIGURE 3. Benzotriazole Coating On Copper

775

● O,.

● 0 ● 0

.“++

‘.‘ .“+”+. ..

IA ‘..,

‘ CU++ .“++. .

- ~ ‘“- ““ “

Cu Cu Cu Cu Cu Cu Cu Cu

FIGURE 4. Substituted Coating On Copper

FIGURE 5. Horizontal Immersion Equipment For OSPS

776