Impact of Quad Flat No Lead Package (QFN) on

Automated X-ray Inspection (AXI)

Andy Pascual Agilent Technologies

Singapore andy_pascual at agilent dot com

Chwee Liong, Tee

Intel Corp Kulim, Malaysia

Copyright © 2007 IEEE. Reprinted from 2007 ITC International Test Conference, Paper 14.3 This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Agilent Technologies' products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions at ieee dot org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.

Paper 14.3 INTERNATIONAL TEST CONFERENCE 1 1-4244-1128-9/07/$25.00 © 2007 IEEE

Impact of Quad Flat No Lead Package (QFN) on Automated X-ray Inspection (AXI)

Chwee Liong, Tee

Test Development Engineering (Malaysia), Intel Kulim, Kulim, Malaysia Andy Pascual

Measurement Systems Division, Agilent Technologies Singapore (Sales), Singapore

Abstract QFN is increasingly being used on wireless cards, handhelds etc. However, QFN unique solder joints pose great challenges for AXI. This paper discusses lack of industry specification for QFN inspection, how AXI methodology was improved to detect QFN solder joint defect, design for inspection and future work. 1. Introduction Today, in the electronics sector the drive towards smaller, lighter and more added features are becoming a norm. As such, QFN (Quad Flat No Lead) packages are increasingly being used on cell phones, handhelds and other portable equipments [1]. Figure 1 shows the volume of QFN will overtake other conventional packages like leaded QFP and array packages in 2010.

Figure 1 Tremendous growth in QFN volume in the future (Source: PRISMARK Nov 2006) One QFN supplier mentioned that they have shipped a billion QFN packages. According to the QFN supplier, the popularity of QFN is due to its superb thermal and

electrical performance characteristics, ease of use and small footprint [2]. X-ray has been widely used in printed circuit board assembly (PCBA) processes for decades now because of its ability to inspect “hidden” solder joints which are either under a component package or shielded by larger components, such as an RF shield. As the complexity and density of boards has increased and test access points needed for in-circuit test reduced, the Automated X-ray Inspection (AXI) machine has emerged as one of the primary test solutions for PCBA manufacturers. AXI not only can detect gross structural solder defects, such as opens, bridging, and tombstones, but it can also flag out solder joint reliability issues such as insufficient or excess solder and solder voiding. AXI also acts as an important “gate” to reduce the number of test escapes reaching end customers and provides information that helps improve front of line processes.

However, as AXI is a vision inspection it is susceptible to high false calls if test program is not fully optimized. High false calls may lead to escapes. A false call is defined as a defect called by the inspection machine but verified as not a true defect by a repair operator while escape is defined as falsely accepted by the AXI but later found as true defect by downstream test such as electrical test.

Since AXI monitors the health of the process, large variation in process will also cause extremely high false calls. As such, high false calls will only decrease the effectiveness of AXI implementation in the manufacturing line. This is because the human operators are required to decide the failure images called by the AXI are real or just false calls. If the false calls are too high, this will tire the human operator who may not go through all the failure images flagged out by AXI. If this happens, there is a potential risk of escapes from AXI station to the downstream stations like In-Circuit Test (ICT) or Functional Test.

Figure 2 shows typical QFN that is being used in the industry. QFN is quite similar to QFP (Quad Flat Pack) but with no leads and large pad in the center. Similar to BGA (Ball Grid Array) packages, QFN also forms solder

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joints underneath the package. Since the solder joints are hidden, x-ray can be used to inspect for any manufacturing defects as well as to feedback any process characterization data to improve the process.

Figure 2 QFN package (Source : IPCA610D) Thus, with the wide proliferation of QFNs today, it is important to understand its impact on AXI. Hence, this paper will discuss in detail the challenges of QFN on AXI, the improvements made to increase its detection rate, design for inspection (DFI) and future work. The AXI mentioned in this paper uses 3D laminography technology. The study presented here was based on the QFNs populated on wireless boards.

2. QFN challenges 2.1 Lack of Clear Industry Standard For AXI to act as an effective process monitoring station, it must be supported by a set of clear acceptance criteria of good and bad solder joints agreed upon by AXI and process engineers. Without this, results from AXI will often be challenged. For example, why certain solder joints are being failed as insufficient. Since IPC A610D standard is widely adopted by industry to define assembly process, this paper refers to it for QFN acceptance criteria. Table 1 extracted from IPC A610D standard describes the acceptability of QFN solder joints.

Table 1 IPC A-610D Requirements on QFN

However, the standard is too generic to be a useful requirement for AXI. For example, minimum side joint length is specified as not a visually inspectable attribute; solder fillet thickness as wetting is evident and solder coverage of thermal pad as not a visually inspectable attribute. These requirements do not take into consideration that x-ray is able to inspect the nature of the solder joints and wetting is evident does not describe how much solder wetting is considered acceptable. Figure 3 shows the variation of QFN solder joints under X-ray. The ones highlighted shows solder is present but whether they are sufficient to be considered reliable is unknown. Unless there is a clear specification on the QFN solder joints acceptance, there will always be argument on whether these solder joints should be accepted or considered as insufficient. Figure 4 shows extensive voids found under the package body which acts as a thermal pad. As void in BGA always provoke serious discussion on the health of the process, should we also be concern on the QFN thermal pad voiding? Since QFN boasts superior thermal performance compare to other packages, does the extensive voids decrease its thermal conductivity; hence causing thermal damage to the device? Figure 5 is an AXI image which shows voiding exist on the terminal solder joints. (Note: The term terminal is used instead of lead as QFN is a no lead package). On some terminals, there are even multiple voids exist. To verify whether the solder voids found on the terminals are real, the sample was cross sected. Figure 6 shows the cross section photos of the voids tally with what was shown by AXI. Again, the same question is posed as to how much void for QFN is acceptable is not mentioned in the IPC A610D standard. Although recent IPC report [3] concluded that there is no evidence that solder joint voiding has any significant

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impact on solder joint reliability, can the same be said on QFN? The test vehicle used in the study did not include QFN. Besides that, the voids as shown in Figure 6 are very large should raise eyebrows as its impact to the solder joint reliability. Therefore there is a need for the industry community to drive for a much clearer specification on what is considered as acceptable QFN solder joint. Although x-ray inspection reveals huge amount of information on the quality of QFN solder joints, lack of industry standards hamper its usage as effective monitoring gate. Thus, one way to define the acceptance criteria for insufficient and voiding is to select the worst case for insufficient and voiding and then subject them to quality and reliability tests such as temperature cycling. For example, if the worst case thermal pad voiding is 30% and fail after temperature cycling, then 30% will be the thermal pad voiding acceptance criteria of the QFN package. On the other hand, if the temperature cycling test passes, then 30% thermal pad voiding is still considered acceptable. This effort is essential to avoid false rejection from AXI.

Figure 3 Potential insufficient solder joints

Figure 4 Center thermal pad voiding

Figure 5 Voiding observed on QFN terminal solder joint

Figure 6 Cross section of pin 9 and 11 showing massive voids on the terminal solder joints 2.2 QFN Solder Joint Open Detection AXI looks for a set of consistent features of a good solder joint type and sets acceptable criteria on them. If any of these criteria is not met, it will be flagged out as defect. A good gullwing joint, for example, has high

Pin 9 Pin 11

VoidPin 9

Void Void

Pin 11

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heel fillet that usually extends above the lead thickness, a low toe fillet and a thin center. Center is the solder underneath the lead between heel and toe fillets. One condition for open joint used by AXI is how high the heel fillet compared to the center. Measured center thickness is subtracted from measured heel fillet thickness to quantify this magnitude. If there is no heel fillet formation, the measurement difference is small and sometimes zero or negative value as illustrated in Figure 7.

(a) (b)

Figure 7 Gullwing heel minus center, (a) good and (b) open joints Heel minus center and other criteria that were derived from good solder joint features are applicable only because of standardization of SMT packages. Commonly used packages have established standards like gullwing, chip, BGA, CGA, PTH, Jlead, and SMT Connectors. They have consistent solder joint features that can easily be learned by AXI. When QFN was initially inspected under x-ray, its terminal solder profile looked like a reversed gullwing solder joint whereby the toe fillet is higher than the heel fillet. Therefore, AXI users normally reverse the QFN CAD pin orientation during test program setup to detect open by using the concept of heel fillet thickness minus center thickness. Open solder joint will show a negative value and vice-versa. Combining this concept with fillet length and fillet thickness, insufficient solder could be detected reliably. However, it was found later that this open detection was not always effective for all types of QFN. In the following paragraphs it will be discussed why the basic techniques for open were not always applicable. Depending on packaging technology, there are QFNs that have terminal pads extending outside its body like the one shown in Figure 8.

(a) (b) Figure 8 (a) X-ray image (b) QFN with outer edge termination These good QFN solder joints have high toe fillets and consistent distance from the heel and center. This is due to the terminal pads extending all the way up the sides of the package. This QFN category is called “Outer-Edge termination” where IPC J STD-001D requires a toe fillet formation as shown in Table 2 on Minimum Toe (End) Fillet Height. Table 2 IPC J STD-001D Requirements on QFN

Toe fillet can vary in formation and size after reflow. This is shown in Figure 9a to 9d. Depending on the surface mount process, toe fillet can be very dense; forming a large convex shape as shown by Figure 9a, Figure 9b, and Figure 9d or just nice wetting profile in Figure 9c. This feature can be used by AXI to separate good solder joints from solder joints that do not have toe fillet formation.

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(a) (b) (c) (d) Figure 9 QFN with outer-edge termination of different toe formation (Source: Amkor Paper on Board Assembly and Reliability Considerations for QFN) AXI allows users to set the allowable deviation from a nominal value of a certain solder feature before flagging out the solder joint as failure. As such, no toe fillet or just slightly low toe formation can be set to trigger an open failure or insufficient solder. Insufficient toe, therefore, can be easily detected. However, since neither IPC A610D nor IPC J STD-001D specifies the required toe fillet height at certain percentage of the termination height, the definition of defect is dependent on the specification from the PCBA manufacturers. Figure 10 shows another type of QFN which does not have outer-edge terminations. This type of QFN is not required to have toe fillets. The toe fillets for this type of QFN may form after reflow but the size and height is not consistent. Figure 11 shows the terminals of this QFN type having variations from no toe fillet at all to thicker than the heel fillet. None consistency of a good joint using toe fillet as a basis leaves AXI to inspect only the heel and the center features. This type of QFN is called “Bottom-Only Termination” which poses the greatest challenge to AXI. Besides that, it is difficult for visual inspectors to accurately spot defects at the terminals, even with the use of magnifying instrument without any means to view underneath the component. Furthermore, Outer-Edge termination types may get categorized as Bottom-Only termination by their manufacturers. Like one QFN supplier that regards the presence of toe after reflow only as additional strength for reliability [4].

Figure 10 Bottom-Only termination QFN

Figure 11 Large variation of toe height for Bottom-Only Termination type of QFN One of the inspection approaches for Bottom-Only termination, which does not rely on toe, is to use heel fillet slope. This is taking the heel profile and measure the highest slope along the heel end of the joint knowing that an open joint will have lower measurement. Figure 12 shows a heel profile and where its slope is taken, while Figure 13 shows a comparison between heel slopes of a good and bare reflow board of one type of Bottom-Only Termination with 20 by 50 mils pad size and 30 mils pitch. A bare reflow board was created from unpopulated board which was paste printed and reflowed. The bare reflow board was used to simulate QFN open solder joints. This board was used as baseline inspection criteria for AXI. Theoretically, AXI should fail all the solder joints on the bare reflow board.

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Figure 12 Location of heel and toe slope on the solder joint The result in Figure13 shows heel slope of good joints was higher than the open joints. However, its variation was too large which could cause high false calls. Although measurements for good solder joints did not overlap open solder joint, the smallest gap between the two was zero. Figure 13 QFN heel slope comparison of good and open joints

Figure 14 shows the toe slopes of the same package used in the previous example on heel slope. Bottom-Only Termination varying toe slopes gave overlapping values between good and open. Clearly, this parameter cannot be used.

Figure 14 QFN toe slope comparison of good and open joints However, adding toe slope to heel slope was found to give a good separation between good and open solder joints. This is the sum of heel and toe slopes of each QFN joint, and is shown in Figure 15. With about 10% gap, this is a promising parameter.

Figure 15 QFN sum of heel and toe slopes comparison of good and open joints As there are many sizes of this package, it was necessary to test this approach on a smaller size terminals kind of Bottom-Only Termination. Figure 16 shows the slope sum comparison of a QFN with half the pitch and pad size of the former type. There is overlapping between good and open. It shows that if the threshold is set at 0.7, AXI will have 80% to 85% detection but 15% to 20% false calls. This slope measurement turned to be an interim solution for the immediate need of AXI. It was not a perfect solution but it gave confidence of some extent in capturing real defects.

Good

Open

Good Open

Good Open

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Figure 16 Slope sum comparison of good and open joints for 12 x 26 mils QFN terminals With the lack of guideline for underneath joints, it became great challenge for AXI to accept large solder joint variations without having high false call rate and at the same time capable of detecting true defects. Therefore, studies were made to get to a level of acceptable false call rate and increase the capability to detect solder defects. 3. Improvements made to the current algorithm

The search for the characteristics of a good QFN solder joint that will differentiate it from a bad joint was not easy. Different pitch and terminal sizes, package types and even component suppliers must be considered. Another requirement is that the surface mount process must be ready to provide samples of defects. AXI had to deal with changes made by these variations in solder thickness profile, joint size and shading. It must be noted that QFN package from different suppliers used gives different shading during x-ray inspection which may contribute to false failures if not considered earlier in the test development process. A change in level of background shading is caused by the different materials used by component manufacturers. Among the common types of solder joint defects, open joint was the big challenge and so this will be the discussion in the following paragraphs of this section of the paper. For every process change, a bare reflow board was produced in order to get samples of open joints. Therefore, all open joint measurements in the charts shown here were from bare reflow board. The parameter settings used in getting those measurements were first validated using real defect examples. As the solder joint

profile from the bare reflow board may not be exactly similar to the real open sample, actual open sample is desirable. Figure 17 shows AXI image of a real manufacturing process open QFN defect. The QFN was slightly tilted causing the solder of those five land pads not to wick up to make contact with the QFN pads. The board was found to fail functional test later.

Figure 17 QFN open solder joint Figure 18a, is another example of open joint but this time it was fault injected. A resistor was placed underneath to tilt the component because the previous open solder joint sample was not easily reproducible. A picture of a cross-sectioned open joint is shown in Figure 18b with a graphic drawing on the right to help illustrate the real scenario.

(a)

(b)

Figure 18 (a) X-ray image of open solder joint by tilting QFN using chip resistor (b) Cross-section showing open solder joint

Good

Open

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Having actual open solder joint samples helps the R&D from the AXI supplier to enhance the algorithm. The defect detection techniques added for open are heel sharpness, joint (fillet) width, center region thickness, and upward curvature. These are necessary to cover the various types and configurations of QFN. Heel sharpness is a variation of heel slope that computes the curvature of the heel edge to take advantage of heel regularity as compared to toe. It helps find real defects but not all. Its measurements of good joints are compared to the open (bare reflowed board) in Figure 19.

Figure 19 Heel sharpness of a good joint is charted against open joints from bare reflowed board The center region thickness and fillet width were considered and gave a very good separation of good from open joints. Figure 20 illustrates why these two features can be used. A good joint is always flat because it is sandwiched by the package terminal and the land pad while an open forms a gentle sloping hill across the pad. Cross sectioned good and open joints, with their corresponding AXI images, showing real scenarios in Figure 21a and 21b.

Good Open Figure 20 Feature of good and open QFN fillet across the pad

(a) (b) Figure 21 Cross-section view along the pad of showing (a) good and (b) open QFN solder joint The charts in Figures 22 and 23 show the separation of good from open joint measurements of each parameter, center thickness and fillet width.

Figure 22 Comparison of center thickness between good and open solder joints

Figure 23 Comparison of fillet width between good and open solder joints

Good

Good

Good

Open

Open

Open

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The upward curvature is fairly effective on smaller size QFN terminals. Figure 24 charts its values, good and open. This data is from 12 by 26 mils QFN terminals. Upward curvature is measuring the concavity formed by the joint along the pad, from heel to toe. A good solder joint normally has a flat region while an open solder joint region has a downward curve.

Figure 24 Comparison of upward curvature between good and open solder joints Among these new parameters, fillet width measurement is the most effective and easiest to set because it works well with today’s commonly used terminal sizes. However, its open joint measurements are too close to good joints and it is not useful on smaller size joints where upward curvature shows good results. For that reason, it is recommendable to use multiple parameters looking at different features to get the best coverage and acceptable false call rate.

4. Design for Inspection (DFI) In order to have effective AXI implementation, it must be able to distinguish good and bad solder joints accurately. This means that solder joint formation of bad joints must look much different to good solder joints. With this in mind, the fillet formation of the solder joint at the end of the terminal can be used as an attribute to clearly distinguish between good and bad solder joints. Figure 25 shows x-ray image of good solder joint formation where there is visible fillet formation while Figure 26 shows x-ray image of bad QFN solder joints formation where there is no visible fillet formation.

Figure 25 Good solder joints with visible fillet as highlighted

Figure 26 Solder joints from bare reflow board with no visible fillet Figure 27 shows clear separation between good and bad solder joints when the measurements are plotted together. This enable the test programmer to set a robust threshold that will able to distinguish between good and bad solder joints.

Figure 27 Clear separation between good and bad (bare reflow board) measurement of the solder joints Therefore, board designers when choosing QFN could choose QFNs with side terminal so that a fillet can be formed at the end of the terminal. This will aid AXI in making more accurate calls.

Good

Open

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5. Summary The paper highlights the increasing trend of QFN usage in the electronics sector due to its various advantages in terms of size, weight and performance. However, the current industry standard on QFN is still inadequate. This has hampered the effectiveness of AXI implementation to inspect on QFN. Also mentioned in the paper is how QFN solder joints present great challenges to current AXI inspection methodology. However, after extensive effort was done to characterize the behavior of QFN solder joints had helped to improve the AXI detection rate of bad solder joints. This paper also presented design for inspection for QFN in order for effective AXI implementation in the manufacturing line. 6. Future Work Needed As QFN is a type of low Z height package, the solder joint formations do not have clear distinction between open and good solder joint. It is predicted more low Z height packages are coming as product getting thinner and smaller. As such, this is going to push the AXI technology and inspection methodology to the limit. In order to protect customer investment on AXI, it is essential for AXI suppliers to continue developing and improving the current inspection methods so as to cater to future packages. It is also equally important to develop good DFI (design for inspection) on either the packages or PCB that will enable AXI inspection to be more effective. Normally, this effort will require close collaboration work among AXI suppliers, package suppliers and OEMs. More characterization work is also needed to define the acceptability criteria of the QFN solder joints. Finally, industry groups like IPC need to update its current standards to reflect the changing needs and advances in the PCBA industry.

7. Acknowledgements The authors would like to acknowledge the following people to make this project successful: Intel team: Chin Chuan, Ng; Tzyy Haw, Tan; Jui Ang, Tan; Kook Hsiung, Ooi; Thomson, Barbara A

Agilent team: Wong, Sam TS; Rivas, Rich; Jessen, Jeremy R, Koczera, Barbara; Liu, Gary Also not forgetting Inetest AXI test support engineers. 8. References [1] C.James “Big Growth for Tiny Packages”, www.purchasing.com/article/CA6361109.html [2] www.amkor.com/news/PressReleases/ showpr.cfm?id=244 [3] IPC Solder Products Value Council, “Round Robin Testing and Analysis of Lead Free Solder Pastes with Alloys of Tin, Silver and Copper,” IPC Solder Products Value Council Final Report, July 2005. [4] S. Ahmer and K. WonJoon, “Board Level Assembly and Reliability Considerations for QFN Type Packages”.