Improved Organic Solderability Preservatives (OSPs)

for Mixed Metal Finishes

Michael Carano and Koji Saeki The trend toward alternatives to hot air solder leveling has been well defined The Asian Rim, led by Japan, has pushed OSP to a leadership position as a final finish. Reasons include environmental and safety concerns over lead, the need for a coplanar surface, lowest possible ionic contamination of the surface, fine-pitch device assembly (e.g. BGAs and flip chip), reliability, and cost.

OSP Technology is environmentally friendly, provides a coplanar surface, and requires very low equipment maintenance. The process is designed for horizontal conveyorized processing. However, vertical immersion systems are also easily integrated into the printed wiring board fabrication process.

Trend Towards Mixed Metal Finishes With the trend for increased packaging density (higher I/O count, finer pitch) has come the use of COB (chip on board), flip chip and TAB (Tape Automated Bonding).

PWBs with mixed metal finishes, i.e. copper pads and other features plated with gold, silver, tin or solder have grown in importance and require special OSPs. A water-soluble OSP, that was capable of protecting the bare copper from oxidation without leaving a film on the other metals, needed to be developed and implemented. Conventional OSP processes based on azoles functioned adequately to protect the bare copper but deposited a film on other metals that interfered with wire bonding and increased contact resistance on gold. The removal of such a deposit from non-copper surfaces would add processing time and cost. It has also been determined that the copper ions that are part of the conventional OSP protective film contribute to ionic contamination. It has been demonstrated that the copper contributes to the staining/darkening of the solder and causes undo build-up of residue on the gold.

Therefore, it was imperative to develop an OSP process that would selectively deposit on the bare copper surfaces only, with low residual ionics.

The Chemistry of the Process

Figure 2. Soldering Process. For this process, a unique imidazole compound was synthesized as the active ingredient of the OSP (US. Patent # 5,795,409). This unique compound is solubilized in water and a nominal of amount of acetic acid. Acetic acid was chosen over formic acid since it is less volatile which makes it easier to maintain a stable concentration and pH range, resulting in a more consistent OSP deposit thickness.

Figure 1. Advancements in Imidazole to improve deposit properties.

This process employs a combination of a phenyl imidazole compound mixed in an aqueous solution with acetic acid, a complexing agent and a water soluble iron compound. The combination of these additives permits the uniform coating of the organic film on the copper without building any appreciable amount on the non copper surfaces. After the printed wiring board has been prepped in an acid cleaner, followed by a micro-etch, the OSP coating is applied to the PWB by either dipping, spraying, or flood coating.

The latest generation OSP active compound is shown in Figure 1. The substituted phenyl Imidazole represents a significant improvement in solderability protection as well as preventing subsequent OSP film deposition on other metals such as gold.

OSP Coating Thickness A common fallacy among OSP users is the thicker the coating, the better the solderability protection. Very thick coatings actually make the removal of the OSP by the action of the flux vehicle more difficult. It has been demonstrated that thicker OSP coatings reduce the spread of solder paste, often leaving bare copper visible and contribute to insulation resistance on gold contacts, and poor solder joint formation.

The nominal thickness of the OSP film produced from this process is between 1500-2500 Angstroms as compared to other commercially available systems producing thicknesses on copper in the range of 4000-7000 Angstroms. The thickness of the coating is determined by the procedure outlined below:

1. Prepare an OSP dissolving solution by diluting 27.8 grams of 36% HCl and 100 grams of methyl alcohol with deionized water to one liter.(reference).

2. Obtain bare copper laminate specimens 4 X 4 cm(S) and lightly clean by pumice scrubbing or microetching.

3. Treat copper specimens through OSP line and coat with OSP.

4. Place specimen in a beaker and pour in approximately 30 ml (V) of the dissolving solution into the beaker. Shake beaker for one minute to completely dissolve the coating.

5. Remove sample board from the dissolving solution.

6. Pour OSP dissolving solution into one cuvette. Pour dissolving solution (reference) into the other cuvette.

7. Use UV Spectrophotometer (single beam is sufficient) with wavelength set at 284 nm. Measure absorbance (A1) of the peak of the above solution in comparison with the blank (reference solution).

8. The coating thickness is obtained from the following formula: Thickness (micron) = 0.105 X A1 X V/S

Process Control To achieve the best performance with this OSP process, several precautions should be taken.

These are:

• Control of pH range • Cleaning of squeezing rollers in conveyorized equipment • Minimize drag-in of water or microetch residues in OSP chamber • Maintenance of active ingredient in OSP solution

Regardless of whose OSP is ultimately used for production, the pH of the chemistry must be tightly controlled within specific limits. If the pH is allowed to increase, the thickness of the OSP coating will also increase. Higher pH values will cause the azole component of the OSP to crystallize out of the solution and even deposit as a heavy residue on the PWB. When an OSP process is operating in horizontal conveyorized equipment, it is customary at the exit of the OSP chamber itself to employ a set of squeezing rollers to minimize drag-out of the OSP chemistry, and to assist in the drying of the coating. It is recommended to spray the rollers with water periodically to avoid build-up of material and board contamination and to aid in OSP removal. If crystals do form, a 5% acetic acid solution applied with a wipe will remove any crystals. However, this OSP Process does not require drying of the OSP coating prior to final rinse. This fact greatly simplifies the operation and improves overall coating uniformity.

Water drag-in and micro-etch residues (especially persulfate based etches) are harmful to the performance of an OSP solution. Persulfate micro-etch requires an acid post clean up prior to the boards entering the OSP chamber itself. Hydrogen-peroxide micro-etch is the process of choice as the chemistry is easier to rinse, and does not require an acidic neutralizer. The azole component of the OSP process should be maintained between 85-105% of its optimum concentration for best OSP thickness control.

Implementation of the Process-Case Study

Figure 3. Typical Circuit for Flip Chip Assembly. A well-known fabricator was a partner in the implementation of this new OSP. This fabricator is also an assembler of flex circuits. The surface finishes currently in use in the facility are:

• Copper with OSP • Solder --Screened/Reflowed --Hot Air Solder Leveled • Nickel/Gold --Electrodeposited(ED) Nickel/ ED Hard Gold --ED Nickel/ ED Soft Gold --Electroless Nickel/Immersion Gold

Flex circuits introduce unique problems to the OSP process such as punched or lased openings in coverfilm material that create “pockets” that can trap liquids during processing Metallic and/or non-metallic stiffeners are occasionally attached to the flex circuit during processing. Any OSP used for these circuits must have good compatibility with a wide variety of stiffener materials such as polymers and alloys of stainless steel and aluminum.

The ideal OSP coating material for the most demanding flex circuit applications, hard disk drives (HDD), would have the following characteristics:

• Thin effective protective coating on copper --Survives multiple bake cycles --Solder readily wets and reflows --Long shelf life • Deposition on polymer and adhesive materials • No deposition on Gold • No adverse effects on stiffeners • No adverse effects on solder

The incumbent OSP process has run successfully with certain limitations inherent to the OSP material and process The following are some of the problems and limitations experienced over the last three years with this process.

Metal Stiffener Compatibility. Because aluminum cannot be run down the line without visible chemical attack, customers requiring OSP have been forced to use stainless steel stiffeners.

Gold Discoloration and Contamination. Because of customer bonding requirements, most disk drive gold pads are essentially 24 karat “soft” gold, and these are especially susceptible to staining and discoloration. Gold pads on processed circuits suffered moderate to severe darkening and discoloration from OSP and copper deposition resulting from the OSP conditioning processes. It was necessary to clean each pad with isopropyl alcohol (IPA), being careful not to degrade the OSP on adjacent copper pads. Even this procedure did not restore the gold to its normal color, causing some customers to reject the use of OSP on their parts.

Figure 4. Excessive OSP Deposits on Unassembled Flex. Excessive OSP Deposition. Process recommendations and equipment design for the incumbent OSP process insure that the relatively soft, moist OSP coating is not thinned by the rinse process. This contributes to a significant ongoing problem with the acceptability of the flex circuits; the drying of residual, unrinsed OSP solution on the parts, causing OSP thickness variation on copper and unwanted deposition of OSP on both the polyimide and exposed adhesive surfaces of the circuit. The problem of removing unwanted deposits is aggravated by drying prior to rinsing, and the fact that there is no fluxing action in these areas. Figure 4 shows examples of these undesirable deposits which can form on an unassembled flex circuit, while Figure 5 shows residual OSP from excessive deposition on the underside of a stainless steel stiffener after customer assembly and cleaning.

Residual OSP Deposits after Assembly and Aqueous cleaning.

Figure 6. Disk Drive Circuit Ionic Contamination (Ionograph).

Ionograph Cleanliness Testing. Ionograph testing is routinely used at the company for testing lot circuit cleanliness. The Benzimidazole formulation has a component which volatizes with time and heat. This component causes parts to have high initial readings on ionograph cleanliness testing, potentially masking other process ionic residues from being observed. Parts coated with OSP must be baked at 70º C for 30 minutes to drive off most of the volatile ionic species before they can be tested on the Ionograph with consistent results. Figure 6 shows a typical example of Ionograph readings on circuits coated with the OSP “as deposited,” versus “after baking.”

Customer requirements dictated the use of aluminum stiffeners, gold bonding pads, and OSP coated Flip-chip bonding pads. As a result of the Flip-chip circuit density, and the close proximity of the gold pads to the Flip-chip pads, the labor-intensive removal of OSP from the gold pads with IPA was not feasible.

The decision was made to evaluate the new Phenylimidazole OSP process. The key performance criteria were:

• Provide a suitable surface for flip chip and SMT assembly, qualified by in-house assembly facilities and customer’s assembly operations. • OSP must leave no discernible film or discoloration on gold surfaces. • OSP process exhibits no attack or darkening of aluminum or stainless steel stiffeners. • No visible OSP deposition on the polyimide base material. Desired overall improved thickness control of the OSP coating on copper. • Desired improved circuit cleanliness after OSP applied. (Ionograph monitoring)

In addition, a “drop in” process was necessary to consider the project successful. The definition of a “drop in” process meant that no equipment or assembly process changes were required. The majority of customers dual source product, and the OSP provided on Company circuits would have to perform similar to a competitor’s OSP finishes, using established assembly process parameters.

Figure 7. Effects of OSP on gold Plated Pads. Initial testing was performed on a Flip-chip test vehicle, to evaluate the solderability performance of the coating. Success with the test vehicle led to actual product testing for multiple product lines; with a variety of surface finishes, stiffener materials, and Flip-chip bump metallurgies. Both stainless steel and aluminum stiffeners were processed through the new OSP with no discernible attack on either metallurgy. Gold pads on circuits processed through the new OSP line actually looked cleaner, and more pristine after processing than before processing. Figure 7 shows three gold test coupons; the one on the left has been processed through the standard substituted benzimidazole OSP, the center coupon is “as plated,” and the coupon on the right has been processed through the new phenylimidazole OSP. As can be readily seen, the coupon on the right has the brightest, lightest yellow gold appearance.

Figure 8. OSP Thickness over Gold.

Figure 8 shows OSP measurements of the thickness of the benzimidazole versus the phenylimidazole over gold on coupons. The phenylimidazole does not need to be dried prior to the final rinse, and therefore is not susceptible to extraneous OSP deposition on the noncopper surfaces. Figure 9 shows an example of a circuit processed through this OSP.

Figure 10. Ionograph Testing of Standard and New OSPs. The new OSP does not have any components in the deposit which interfere with an Ionograph cleanliness test, precluding the requirement to bake before testing. Figure 10 compares Ionograph readings between circuits processed with benzimidazole, unbaked, and baked; and phenylimidazole, unbaked. Assembly Testing

Figure 9. Circuit Processed Through Phenylimidazole OSP.

Assembly testing began with the Flip-chip test vehicles without changing the assembly process. The Flip-chip process in use at the company is a two step process, with the surface mount devices assembled first with a reflow and clean operation, followed by the Flip-chip assembly, with its associated reflow and clean operations. The results were good, the solder wettability and spread was equivalent, with Flip-chip standoff height within 5micron of that obtained with the standard OSP. Figure 11 shows a cross section of a Flip-chip assembly using the new OSP coating.

Table 1. Testing Performed on HDD Circuits with Phenylimidazole OSP.

The evaluation was then expanded to include SMT and Flip-chip assembly, reliability testing and solder joint studies on two actual products. The new OSP showed no differences from the standard OSP for SMT solderability, Flip-chip die bump wetting, and Flip-chip underfill flow. Visual inspection was good, with no rejects after assembly. Two parts failed functional test for die related problems, not related to the OSP. The additional tests and results are listed in Table 1.

The trials demonstrated that no process changes were necessary to implement this OSP, and that yields test results were comparable to those using the standard OSP process. Subsequent assembly testing by HDD customers proved the interchangeability of this new OSP with Benzimidazole type OSPs, as well as the increased cleanliness and yields resulting from the lack of extraneous deposition of OSP material.

Summary and Conclusions The company’s fabrication division recognized the following benefits:

• More consistent and uniform OSP coating thickness

• No extraneous coating on non-copper surfaces

Figure 11. Example of Flip-chip Die Attach with Phenylimidazole OSP.

• Wider process operating window

• Lower levels of ionic residues as compared to previous OSP process

• More versatile process, able to process circuits with multiple metal finishes and

stiffener materials, without tarnishing or degrading those other finishes

The company’s assembly division and its other customers found several benefits including:

• More consistent product quality, less incoming ionic and visual contamination

• Thinner overall OSP coating thickness, promoting excellent solder paste spread

• Ability to process circuits with gold plated surfaces without discoloration

• Ability to process aluminum stiffeners through OSP process

• Process is suitable for flip chip assembly

This latest generation OSP based on aryl-phenylimidazole shows excellent stability even through multiple thermal cycles and selectively deposits on copper. The thinner coating promotes excellent solder paste spreadability when compared to thicker OSP coatings based on benzimidazole. Finally, the absence of copper in the OSP solution and thus in the organic film has been shown to improve the level of ionic cleanliness significantly.

ADDITIONAL INFORMATION

Michael Carano is the vice president of marketing and strategic development for Electrochemicals. His email is mcarano@electrochemicals.com.

Koji Saeki is the product manager for Shikoku Chemical Corp. His email is saekik@shikoku.co.jp