Test and inspection in lead-free manufacturing

The industry is predicting higher defect levels during the transition from lead to lead-free. It is expected that defect levels for opens, shorts (bridging), voids, and misalignment will increase when going to lead-free. Tin-silver-copper alloys do not wet the surfaces being soldered as well as tin-lead solders, so solder bridges will be less likely to clear themselves and parts will not self-align as well. For insufficient and excess solder the industry expects defect levels to be approximately the same as they are today.

Figure 1 shows defect levels from HP Loveland site when they went to a no-clean process. The shop built a high-mix of medium- to low-volume boards for multiple product lines, so there were a large variety of components, processes, and materials. Defect levels increased by more than an order of magnitude and it took around two years to get defect levels down to the
level they were prior to going no-clean.


A summary of transition issues that will be especially important to test and inspection are:
• Many are likely to see higher defect levels.
• New and traditional defect types make it important to look for defects all the way from solder paste, placement, before and after reflow, after wave and then electrically to test the product through In-Circuit test (ICT) and Functional test.
• Increase of process issues and increased need for inspection focused on process control (SPI and AOI-pre reflow). Since we also expect higher defect levels at the end of the manufacturing process, test and inspection for defect containment will also be more important.
• Because of the reduced number of allowable repair attempts, test methods with very high diagnostic resolution will be important.


Different wetting characteristics
The biggest impact from lead-free solder on defect levels and the defect spectrum is caused by the different wetting characteristics of this alloy. Wetting characteristics of molten solder and the surfaces it is joining determine how well the solder covers pads and leads, and determines the shape of the solder joints. The wetting force of lead-free solder is not as strong as for the tin-lead solder. Early production and experiments have shown that this will impact defect levels and defect spectrum, and the following are the most obvious examples seen.



The first is solder pad coverage. Because of the lower wetting forces of lead-free sometimes the full pad is not covered with solder after reflow. This can be seen in figure 2. This illustrates two cases of empty pads where no components are placed. The pad to the left is tin-lead solder and you can see that it is fully covered with solder. The pad to the right is lead-free solder and solder is covering only the right side of the pad. In most cases this would not be considered a defect and would not negatively impact the manufacturing of the PCBA. However there may be an impact on in-circuit test (ICT) and/or functional test. If this is a test pad and the test probe is supposed to hit solder, because of the incomplete solder pad coverage it may hit raw copper instead. From experience, test probes do not make good contact with raw copper and this will result in a fixture contact problem, lowering the yield at ICT and/or functional test.

Another potential problem is with bent leads. If a pin is slightly bent the stronger wetting forces of the tin-lead solder compensate in many cases and an acceptable solder joint is formed. When lead-free solder is used there will be more cases where an acceptable solder joint is not formed. It will either be an open solder joint or it will be less reliable than needed.

The wetting forces also help readjust slightly misplaced components during the reflow process. Again because of the reduced wetting force the lead-free process will be less forgiving. This will increase the importance of placement accuracy and of pre-reflow inspection, or result in higher defect levels of misplaced components.

Also the wetting forces of solder help if there is solder paste between two pads potentially creating a solder bridge or electrical short between these two pads. For the tin-lead solder the wetting force, in most cases, draws the solder paste to the pads, thus eliminating the solder bridge. For the lead-free case this is less likely to occur, resulting in more shorts.


Summary of issues

SMT: Higher reflow temperature will stress components and the PCBA more. Logistics of lead-free and lead components was discussed. It is very likely that defect levels will increase. All defect types that occur today should be expected but there might be a modification to their frequency. Tombstoning, misalignment, opens, shorts and voiding have increased.

Wave: The wave soldering machines are likely to be retrofitted or replaced due to higher corrosiveness of the new alloys. Tin forms intermetallic compounds with the iron components of the wave system, resulting in contaminated solder. An increase of insufficient barrel fill is expected due to higher temperature needs for the new solder alloy. More solder-bridges have been reported compared to a standard tin-lead process. Another concern is additional failures to the reflow solder joints, such as joint separation, also due to the higher temperatures at the wave process.

Rework: A new concern with lead-free is that fewer repair attempts may be allowed due to the higher temperatures and that those higher temperatures could cause damage to the PCBA and neighboring components.

Reliability: Early studies indicate that the lead-free solder joint generally has similar reliability than the lead version. However the higher temperatures may impact component reliability. Tin whiskers, which grow over time, are significantly higher [2] for lead-free alloys than the tin-lead alloy. This is a long-term reliability issue and is unlikely to be detected before the product ships.

Logistics: A key issue is how to separate lead-bearing and lead-free components. This applies to many areas in production including many repair areas.

Other: Most lead-free production to date has mainly been done on lower complexity boards that are small in size with few different component types, and are built in high volumes allowing process optimization. It is expected that higher complexity boards with a large variety of component types and built in higher mix environment will be initially more difficult to tune to an optimal lead-free process.






The last example is from the wave soldering process, where throughhole pins are soldered. Because of the lower wetting forces, the solder will decrease its ability to fill the hole properly resulting in insufficient barrel fill. This can be seen in figure 3, which is an x-ray image of one through-hole component. On one side of this component no solder has raised up in the hole.


Defects from lead-free manufacturing



Many examples of lead-free soldering exist today. Most production experience of lead-free is from high volume consumer products. These boards tend to be lower in complexity if we consider component count, and solder joint count per board. The variety of component types is also typically lower than bigger boards manufactured in lower volumes. The following is production data from one major CM that manufactures two similar board types, one in tin-lead and one in leadfree. The boards are for consumer electronics and the lowest pitch for both board types is 20 mils (BGA, gullwing, and SMT connectors) and the smallest chip component on both boards is 0402. Number of components per panel is around 1,300, and number of solder joints per panel is around 3,000. These boards have been manufactured in high volumes – more than 85,000 boards for the tin-lead type and more than 60,000 for the lead-free type. Figure 4 shows the defect levels and defect spectrum for these two very similar boards. Note that the figure only shows defect types impacted by the lead-free process.

As can be seen in figure 4 the greatest increase was found in tombstones. Note that the majority of chip components are 0402s. A significant increase of solder opens and component misalignments can also be seen for the lead-free board. Note also that bridging/shorts decreased for the lead-free board. This is opposite to what is to be expected and may just be a coincidence or because the lead-free board was under close control, supervised by the process engineers with most experience and manufactured with the most up-to-date manufacturing equipment.

The DPMO (defects per million opportunities) level for the tin-lead board was well below 100 DPMO, a very good number. Note that the DPMO level more than doubled for the lead-free board, however it is still very impressive. These DPMO values are calculated on a component basis.



Figure 5 is an x-ray image of another example of a lead-free board. It illustrates the wetting problem discussed above. The two pins in the lower left corner are open. The solder joints to the right show significant wetting problems, and numerous voids can also be detected in the image.

In summary, these are observations of lead-free defects mainly from production of low complexity boards, prototype runs and lead-free experiments:

• Voiding: Significant increase in voiding, however it is still debatable which voids are defects or not.
• Tombstones: Significant increase in some cases.
• Insufficient barrel fill: This is for through-hole components that have been through a wave or selective wave process.
• Bridges: Mainly in the wave or selective wave process.
• Tin whiskers: This is a long-term reliability issue and is unlikely to show up during production testing.


Test strategies for lead-free
The first step before implementing lead-free manufacturing is to establish a good picture of the current tin-lead process. What are the current defect levels and defect spectrum? From a test/inspection point of view, where are the bottlenecks? Most manufacturing sites already have this in place, but if not, it should be established. Some understanding of levels of potential defects and process indicators are also an advantage to know.

For a site with many manufacturing lines it is recommended that only one line be switched over to lead-free first. Very tight test and inspection should be implemented for this line and engineering resources should be available to analyze any new systematic issues that may evolve. As has been stated, there are many new issues when switching to leadfree. The process window is narrower due to the wetting issues, increased reflow and wave solder temperatures and component specifications on maximum allowed temperature. Implementing good process characterization steps is recommended and can be done with solder paste inspection and pre-reflow inspection. It is also important to have good test strategies after reflow to capture all defects. Data gathered from test and inspection should be used to improve the process and to eliminate systematic defects. If faults and defects are increasing significantly, adding test and inspection capability should be considered. When all issues have been resolved and defect levels and quality levels are acceptable, switching lead-free manufacturing to the next line should be considered.

Note that there are likely to be big variations from site to site and between board type and board type. Some sites may experience very few issues and defect levels will be overall the same, while other sites may have significant issues and significant increases in defect levels. Also variation in issues and defect levels can be seen from board type to board type. Some boards may switch over to lead-free without any problems, while others may create significant problems. The bottom line is that test and inspection will be significantly impacted if defect levels increase and the key is to be prepared for a potential increase in defect levels and hope for the best. Switching to lead-free is a significant process change.


Impact on test and inspection methods and equipment
• Solder Paste Inspection (SPI)

Robust process characterization is needed for lead-free processes, as there are more unknowns with this new material. Using 3D SPI allows for quick optimization of print parameters and characterization. From studies from the past 10 over years for tin-lead solder paste, solder paste volume is found to be linked to long-term joint reliability. The same applies to lead-free solders and with lead-free processing causing increased reliability questions and concerns; the need for 3D SPI is even more obvious. Placing equal solder volumes is also known to prevent tombstones. The main advantages with solder paste inspection systems are that it finds potential defects and process indicators, which will lead to fewer defects and faults. It is even more important to control the process when switching to lead-free.


• AOI pre-reflow and post-reflow

There are some differences in the visual appearance between tin-lead and lead-free solder joints that could impact AOI systems. Because leadfreesolder has a lower wetting force than tin-lead solder, it results in a slightly different shape of solder joint. Also lead-free joints are grainier and appear slightly duller than traditional solder joints. To investigate if this would impact the performance of AOI systems, the National Physical Laboratory (NPL) in the UK did a study of six different vendors of AOI systems. The result was published 2002. The study found that today’s AOI system can inspect lead-free boards and solder joints. False calls for tin-lead and lead-free boards and joints were also very similar. The results varied slightly for the six different vendors’ AOI equipment; the conclusion was that most AOI systems can inspect lead-free boards today without any problems.


• AXI (Automatic X-ray Inspection)

X-ray inspection uses different materials’ impedance to x-rays to create an image that a computer can analyze. Now with lead-free will xray continue to work? The answer is yes! The materials used in lead-free alloys still will give enough contrast to give good x-ray images of solder joints. Figure 6 shows an x-ray image of a board using the normal tin-lead
alloy. Figure 7 shows the same board type with a lead-free alloy. As can be seen there is not much of a difference. Note that the human eye can only differentiate between sixteen levels of gray. Accurate measurements will give a 15-20 percent thinner value for lead-free as for tin-lead alloys. This can be compensated if accurate thickness measurements are needed. However, the shape of the solder joint shows how most defects are detected, and those shapes are basically the same for tin-lead and for lead-free solder joints. So x-ray equipment will work with lead-free boards.






• ICT

ICT relies on good contact between the test probe and test pad. This is achieved by using a hard, sharp probe hitting a soft, solder-coated test pad. The probe penetrates the soft solder on the pad, breaking through any contaminants like lead oxide and flux residue. The depth of penetration is a function of the yield strength of the material and the sharpness of the probe. The deeper the penetration, the better the contact. Tin-lead solder has a yield strength of about 5000 psi and, coupled with an 8-oz sharp steel or beryllium tipped probe, this makes good contact.

HASL (Hot Air Solder Level) provides a solder-coated surface to all locations on the bare PCB. If switching to OSP (Organic Solder Preservative), this step is removed. Now, solder is only applied to locations defined by the stencil in the paste machine. Typically, these locations did not include test pads. If no action is taken, test pad targets will be raw copper. Raw copper has a yield strength an order of magnitude higher than lead-based solder, and it is so thin that the test probe may damage the pad. Plus, it will rapidly build up an oxide layer during and immediately after re-flow. As a result, contact will be poor between the fixture and the board under test. If at all possible, one should never probe raw test pads. All ICT testability guidelines indicate this.

The solution is to make sure that the solder paste stencil includes openings for test pads. This will provide solder on the pads restoring contact. Lead-free alloys have a yield strength that is less than most lead-based solder alloys used today, so the contact performance should be similar. In addition the tin oxide is electrically conductive so a layer of oxide on the solder joint will not be a problem for probing the lead-free solder. Compare that with the lead oxide that works like an insulator and must be penetrated by the test probe to make electrical contact.

Due to poorer and slower wetting of lead-free solder, stronger fluxes may be used to promote better wetting. There is some indication that the flux residue from lead-free solders may be harder and more difficult to penetrate than for tin-lead solders due to the elevated soldering temperatures. It is recommended to work with solder paste vendors on a solder paste mix to minimize the effect of these contaminants.

The ICT DFT guidelines can be found in the latest SMTA document that has been developed by industry representation of both ATE vendors and ATE users. It also contains DFT guidelines for AOI, AXI and Boundary-Scan


• Functional test

A new issue with lead-free is the higher reflow temperature of the alloy. This may restrict the number of repair attempts that are allowed due to potential damage to the board and adjacent components. The first thing to minimize repair attempts at functional test is to do a better job at process test so extremely few manufacturing defects escape to functional test. In addition fault isolation at functional test is often done by a shotgun approach, where one starts replacing the most likely component to be defective. If that does not work, the second most likely component is replaced.

Writing the functional test program with better diagnostic resolution would be an advantage from this point of view, but may not be technically possible or economically justifiable. Using software solutions that minimizethe amount of shot gunning needed would be recommended.

Board test functional test is done through edge connectors, a bed-ofnails, or a combination of both. If a bed-of-nails is used the same issues and recommendations apply as for ICT fixturing described above.


Conclusion
The switch to lead-free is a major process change and will impact PCBAs for products that are mandated to switch to lead-free before July 2006 as well as PCBAs for exempted products. Since not all components will be available in a lead-free version, while others will be available only in a lead-free version, all PCBA manufacturing will be impacted. From a test and inspection point of view, defect levels in many cases will increase significantly and also the defect spectrum is likely to change somewhat. Higher defect levels and a changing defect spectrum mean that more attention to test and inspection is needed. Studies and early experiments show that test and inspection systems and methods are ready for lead-free. Additional data gathering of defect levels and defect spectrum for production lead-free boards is needed.


REFERENCES
1. John H. Lau & Katrina Liu, “Global Trends in Lead-free Soldering”, Advanced Packaging February 2004
2. NEMI http://www.nemi.org/projects/ese/tin_whisker_activities.html
3. NPL Document MATC(A)119, July 2002 http://www.npl.co.uk/
4. SMTA Document TP-101C, 2002 http://www.smta.org


This paper was first published in the ECWC 10 Conference at IPC Printed Circuits Expo, Apex and Designer Summit 2005, Anaheim, Calif., Feb. 22-24, 2005.