|A publication of the National Electronics Manufacturing Center of Excellence||
Michael D. Frederickson
The following Tech Tips address lead-free evaluation or certification test planning. Implementing lead-free solder processing often requires evaluation of solder materials or certification of the assembly process. With the seemingly endless number of variables, including solder alloy, reflow profile, aperture reduction, flux types, and component finish, certification can become an unnecessarily arduous ordeal.
1. Selecting a test board
An alternative is to use a standardized test board or coupon, which has a number of advantages over a production board. The use of standardized test boards incorporates a level of reproducibility. Bare test boards can be easily screened, quality checked, and compared to previous test runs. Also, results from testing can be easily evaluated against previous tests and industry standards. Because standard test boards and coupons are used regularly, test facilities are more likely to have appropriate test apparatus and techniques available. This reduces test preparation time and costs and increases accuracy. Many standard test boards are daisy chained, allowing for electrical measurement using common laboratory testing equipment. In many cases, the test board can be fitted with a connector or contacts that allow for continuous monitoring of electrical characteristics. Tracking failure location is also easier. Most have layouts that make testing or inspection extremely efficient. For example, the EMPF-009 test board has a number of discrete components aligned in a row for quick solder inspection and to increase the number of solder joints that can be cross-sectioned at one time.
Standard test boards are available from in-house test facilities, outside laboratories, consortiums, and from component manufacturers. When selecting a standard test coupon, it is essential to match the board level technology with the level of technology in day-to-day production. It is also advantageous to select a test board which has some areas with a slightly higher level of technology. When that new technology is introduced to production, retesting will be unnecessary. Fine pitch components, chip scale packages, and small chip components are some of the more common advanced technologies that are added to test boards to increase processing capability.
2. Selecting test variables
Some of the process evaluation variables used for lead-free conversions include:
The most common lead-free evaluation or certification will include multiple board surface finishes, one solder paste alloy for surface mount, one solder alloy for wave soldering, one flux type, and multiple suppliers. For example, one analytical lab customer that was converting to a lead-free process conducted research before they ordered testing. They supplied ENIG and ImmAg finished boards soldered with a SAC 305 alloy for surface mount and SN100C alloy for through-hole. The manufacturer had been using a no-clean flux chemistry in the past and saw no reason to change their cleaning process. They continued with a no-clean flux chemistry in both wave and surface mount operations. Two manufacturers supplied solder paste for their evaluation. They manufactured test boards using each of the manufacturer’s recommended profiles and continued to solder with nitrogen as they have with their SnPb assemblies. The assembler was able to assess which bare board surface finish and paste supplier had the best performance while also certifying the assembly process using the SAC 305 and SN100C alloys.
Test matrices can easily get out of control by introducing too many variables. However, research into what has been performed to date from other companies, consortiums, and test labs can eliminate many of these variables. Results from many studies are available regarding the performance of various surface finishes with the multitude of solder alloys currently listed as environmentally safe alternatives to SnPb. The poor performers can be eliminated from the test plan, thus reducing time and cost. Only relevant test parameters must be considered for testing. For example, nitrogen use may not be an option for some low-cost operations. Taking the time to research and discuss test planning with those that have experience, prior to organizing a test matrix, can be the most significant step toward a successful lead-free conversion testing and process qualification.
These are only a few of the considerations for lead-free conversion testing and certification. In a future issue of EMPFasis, evaluation techniques and criteria, such as optical inspection, thermal cycling, X-ray inspection, and cross-sectioning, will be discussed.
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