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Lead Free Challenges on Automated X-ray Inspection (AXI)

Koay Hoon Chiow Tee, Chwee Liong Intel Kulim, Malaysia

Abstract Converting to lead free process by every PCBA manufacturer is imminent to ensure their “survival” in the industry. So far X-ray provides effective structural test solution for tin-lead process but will x-ray continue to play an important role in test when lead, a key element in the solder composition which attenuates x-ray penetration is being phase out from manufacturing line? This paper highlights key challenges of lead free on Automated X-ray Inspection (AXI), observations, limitations and recommendations to overcome these limitations.

Introduction X-ray has been widely used in PCB assembly process for decades because of its capability to inspect “hidden” solder joints which are either under component package or shielded by larger components, such as RF Shield. As the complexity and density of the board increases (significant reduction of in-circuit test access points, or coverage), Automated X-ray Inspection (AXI) machine has emerged as one of the primary test solution for PCBA manufacturers. AXI machine not only shows its capability in detecting gross structural solder defects (such as open, bridging, tombstone, and etc) but also solder joint reliability defects (such as insufficient, misalignment, excess, voiding and etc). AXI also acts as an important “gate” to reduce defects escapee to end customer and improve front of line processes and product quality. With pressure from environmentalist and European Union Legislation, lead will be restricted from being used in electrical and electronic equipment by July 1, 2006. As we know, x-ray was used based on the concept of differences in x-ray attenuation by different elements. In the case of tin-lead solder composition, lead being the heavier element greatly attenuates x-ray thus forming a darker image.

Therefore, many questions are being asked on the effectiveness and risks posed when lead is being removed in a lead free environment. For example, what are the changes needed if AXI does not perform as well as in the tin-lead environment? Is AXI ready for lead free process? To answer those questions, we feel that AXI equipment manufacturers need to consider 3 main factors below: 1. Lead Free Solder Thickness vs Gray Level

Calibration 2. Lead Free Solder Joint Profile vs Test

Algorithm 3. Lead Free BGA Voiding Measurement

Accuracy In this paper, these factors will be studied using 3D AXI. The test vehicles used are of mini-PCI form factor. There are 4 boards in a panel with more than 4000 joints per panel.

Lead Free Solder Thickness vs Gray Level Calibration

The 3D AXI establishes focal plane which allow the programmer to check the characteristics of the solder joints at different level of height. This is also known as slices. By doing this, the 3D AXI is able to differentiate x-ray images between top and bottom solder joint and thus avoid image overlapping issue like in transmissive technology (Figure 1). The ability of slicing will allow solder height at particular interested region to be measured. Solder height measurement can be calculated by converting gray level to solder thickness with reference to a solder thickness table (thicker solder joint will result in higher gray level value).

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Figure 1: Transmissive vs Laminography X-ray Images (Source Agilent Technologies Inc) Process of generating the solder thickness reference table involve a calibration board which contains of several different thickness solder stripes and powerful calibration software. A thickness table which correlate gray level (0-255 scale) and solder thickness will be generated by exposing known solder stripes thickness calibration board under x-ray to obtain correlated gray level values (Figure 2). Figure 2: Solder Thickness vs Gray Level Thickness Table (Source Agilent Technologies Inc) Due to different chemical composition, x-ray absorption by lead free solder joint is lower (lower gray level value) compare to tin-lead solder joint of similar thickness. By using tin-lead thickness table, lead free solder joint is correlated to lower solder height measurements and thus impact the accuracy of the AXI equipment in detecting defects such as open and insufficient solder. For instance, some manufacturers define open solder for gullwing joint as solder height at heel region minus solder height at center region (Figure 3).

Figure 3: Good vs Open Gullwing Solder Joint Profile Good solder joint will have higher delta value (higher heel – lower center) compare to open/floating joint (lower heel – higher center). From the study conducted using tin-lead thickness table, thicker solder joint will have larger error in solder height measurement compare to thinner joint (Figure 4) when correlating gray level to solder thickness. Therefore, the resulted delta value for good lead free joint will be lower and nearer to floating joint (compare to tin-lead process) as heel is having larger error compare to center region. As a result, this caused AXI ineffectiveness in detecting real open solder defect (Figure 5). Figure 4 : Measurement Difference vs Solder Thickness (Source Agilent Technologies Inc) Figure 5: Good vs Floating Gullwing Joint Open Value for Tin-Lead and Lead Free Processes

HeelHeel

CenterCenter

HeelHeelHeelHeel

CenterCenter

Good JointsGood Joints

Open Open JointsJoints

TinTin--Lead ProcessLead Process Lead Free ProcessLead Free Process

ThresholdThresholdGood JointsGood Joints

Open Open JointsJoints

TinTin--Lead ProcessLead Process Lead Free ProcessLead Free Process

ThresholdThreshold

S o ld e r T h ic k n e s s v s . % M e a s u re m e n t D iffe re n c e

0 .0 0 %

5 .0 0 %

1 0 .0 0 %

1 5 .0 0 %

2 0 .0 0 %

2 5 .0 0 %

0 2 4 6 8 1 0 1 2

So ld e r Th ic k n e s s (M ils )

% M

easu

rem

ent

Dif

fere

nce

:

Sn

/Pb

vs.

SA

C39

6

3

Similar explanation applies for AXI inaccuracy in detecting insufficient solder reading measurement. Basically, certain percentage below the average solder height (for heel, center and toe) is set to determine whether a solder joint is failing for insufficient solder. With lower solder height reading caused by inaccurate thickness table, AXI will result in over-rejects and thus caused unnecessary rework cost. To overcome these inaccuracies in detecting solder joint defects, AXI equipment manufacturer needs to provide an accurate thickness table for correlating lead free solder joint gray level against solder thickness. This can accomplish by either having a lead free calibration board or a formula to offset existing tin-lead thickness table to correlate lead free gray level to solder thickness. Lead Free Solder Joint Profile vs Test Algorithm Structural defects such as voids, insufficient and open solder are commonly found in lead free process. These defects are mainly contributed by lead free solder chemical properties such as high melting points and surface tension. Therefore, lead free solder will form lower solder wetting and higher number of voids formation at solder joint compare to tin-lead solder. An example of different solder joint profile formed at lead free process compare to tin-lead process is shown as in Figure 6.

Figure 6: X-ray Images of Tin-Lead vs Lead Free Solder Tin-Lead solder joint have better wetting solder profile compare to lead free solder joint due to tin-lead having lower surface tension compare to lead free solder. Though with weaker wetting, lead free is still able to provide good contact without any reliability issue and is within specification limit.

However, AXI will define lead free solder joint as a defective joint as it is not having the same solder profile as tin-lead solder joint. To overcome this false reject problem (pass joint but identify as fail by AXI), AXI programmer needs to redefine the region of interest at solder joint. For example, heel must be located at highest peak area of gullwing joint (heel for lead free joint will not be close to the lead as compare to tin-lead solder) and center must be located in between heel and toe peaks. Besides that, the average value of solder thickness needs to be reprogrammed to avoid insufficient false reject as average solder thickness for lead free solder joint is lower. However, there are false rejects that AXI programmers cannot resolve due to test algorithm limitation. Two main false rejects scenarios encountered by AXI are as below:

• Low solder height/thickness at interface between solder joint and component lead or terminal

• Voids formation in solder joint These scenarios result in similar solder joint profile as open joint, which unable AXI to differentiate between good and open solder joints (Figure 7). Figure 7: False Rejects due to Voids and Low Heel Formation at Lead Free Solder Joint Good solder joint will have higher solder heel than center region; bad/open solder joint center region will be higher or equivalent solder height at heel and center region. AXI open algorithm is based on the difference between heel and center height with

Open Solder DefectOpen Solder Defect

VoidVoid

Solder Joint with VoidSolder Joint with Void

Open = Open = HeelHeel -- CenterCenter

Low Heel JointLow Heel Joint

Void/Open Joint Side ProfileVoid/Open Joint Side Profile

Open Solder DefectOpen Solder Defect

VoidVoid

Solder Joint with VoidSolder Joint with Void

Open = Open = HeelHeel -- CenterCenter

Low Heel JointLow Heel Joint

Void/Open Joint Side ProfileVoid/Open Joint Side Profile

Open Solder DefectOpen Solder Defect

VoidVoid

Solder Joint with VoidSolder Joint with Void

Open = Open = HeelHeel -- CenterCenter

Low Heel JointLow Heel Joint

Void/Open Joint Side ProfileVoid/Open Joint Side Profile

Tin-lead Lead Free

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good joint having larger delta value than open joint. Voids and low heel formations (due to lead free solder joint characteristic) will give a solder profile which is identical to open solder joint. From a recent study conducted on 160 wireless network card boards, total false rejects increased by 3 times with 79% of false rejects due to voids (Figure 8) and lower solder wetting at interface between solder and component lead/terminal. Table 1 shows that lead free solder thickness is always lower than tin-lead solder thickness at resistor, capacitor and gullwing joint. For gullwing joint component, heel thickness for tin-lead joint is significantly higher than lead free joint by 23.85%. Table 1: Tin-Lead vs Lead Free Solder Height Comparison

Tin-Lead Lead Free Component Type

Parameter Mean Std.Dev. Mean Std.Dev.

Resistor 0402 1.71 0.26 1.51 0.23

Capacitor 0402

Pad Area Solder

Thickness (mils) 2.05 0.37 1.73 0.30

Gullwing Heel

Solder Thickness

4.31 0.32 3.48 0.37

This scenario will cause high false rejects which indirectly results in higher escape rate as AXI depends on operator verification. With higher false rejects, the numbers of joints that need verification will also increase, and thus increasing risk of operator making mistakes. Therefore, AXI equipment manufacturers need to overcome this problem by having different type of calculation other than just depend on heel – center measurement to detect open failure. Other methodologies or algorithms such as heel or toe slope (good joint will have steeper slope than open joint) need to be explored further to accommodate for lead free solder joint profile (with lower wetting and voids formation) and thus reduce false rejects.

Figure 8: X-ray Images of Lead Free Solder Joints with Voids Formation Lead Free BGA Voiding Measurement Accuracy

According to IPC-A-610C, void is considered as a defect in BGA component if the voiding is more than 25% in the ball to board interface. The 3D AXI allows slicing to focus on interface between ball and board, and voiding area can be measured accordingly. In tin-lead BGA solder joints, a study was conducted to correlate void diameter (at micro-via pad) measured by 3D AXI against cross-section data. With 13 boards crossed section, the data showed that 3D AXI void diameter measurement is correlated to cross section data, with approximate 1 mil difference (Figure 9). Figure 9: Void Correlation between X-ray and Cross-Section Images On the other hand, similar study was conducted on lead free BGA solder joints and results indicate a high increase in false reject. Even with no voids present, 3D AXI will provide voiding percentage readings of up to 10%. This inaccurate voids measurement by 3D AXI is primarily due to lower gray scale level value from lead free solder joints. The lower gray scale value produces higher noise level when measuring voids because 3D AXI machine consider a pixel as void if the pixel is having lower gray level compare to adjacent pixel or a defined gray value.

Voids Voids

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As discussed previously, lead free process will induce more voids formation, and this includes BGA solder joints as shown in Figure 10. Figure 10: X-ray Images of Lead Free BGA Solder Joints vs Tin-Lead BGA Solder Joints With higher number of void formation for BGA lead free and with inaccurate void measurement by 3D AXI, number of false rejects increase tremendously compare to tin-lead BGA joints. Hence, we feel that the existing algorithm used to capture voids in BGA is not robust enough.

Conclusions and Recommendations Lead free solder has become the “must go” process in PCBA manufacturing process. To cope with such development, AXI plays an important role not only in detecting solder joints defects, but also to help front of line process to understand further on lead free process and technology capability. However, existing AXI and test methodology only shows its capability in inspecting tin-lead solder joint but not in lead free solder joint. To enhance AXI value in lead free process, AXI equipment manufacturers need to start focusing on improvement by considering factors such as lead free thickness table for correlating x-ray gray level to lead free solder joint thickness; test algorithm to compensate for lead free solder joint profile, which is significantly different with tin-lead solder joint profile; BGA voiding measurement accuracy by reducing the noise level incurred by lighter gray level value by lead free joints. Without proper improvement in those areas, AXI performance will significantly degrade (with higher false rejects and escape rate). To stay competitive, AXI equipment manufacturers will need to embrace the challenge to qualify their machine for lead free

process and thus ensure x-ray technology continue to emerge as one of the best structural test solution provider.

References 1. Bernard, D. and Ainsworth, S., “Comparing Digital and Analog X-ray Inspection for Component Analysis”, www.smtmag.com, June, 2004. 2. Roth, H., “X-ray Inspection of Lead-free Solder Joints”, www.smtmag.com, July, 2004. 3. Rice, M., Johnson, S., and Leinbach, G., “3-D Techniques in SMT Test”, www.evaluationenineering.com, November, 2004.

Lead Free Tin-LeadLead Free Tin-Lead