Issue link: https://iconnect007.uberflip.com/i/1305670
NOVEMBER 2020 I SMT007 MAGAZINE 81 other hand, is time that all components being monitored see that temperature. It makes a big difference when you measure the temperature of BGAs— especially the temperatures of inner and outer BGA balls.) Handling and control of moisture-sensitive components and tracking of their exposure times is another issue that is worth digging into since very few companies do it correctly: • What is the handling procedure to prevent baking (when exposure time has expired) of moisture-sensitive BGAs? • How many times do you bake moisture- sensitive components (only once is allowed for some MSL levels)? It is not easy to detect the seriousness of the defect in a company during the course of a short audit. But finding the ratio of bridging to opens over the last six months can give you a good idea about the extent of field failures you may see. About six times more shorts than opens are a good sign since there is prac- tically zero chance that shorts would escape any inspection or test. Opens, on the other hand, can easily escape inspection and test and will come back to haunt you after a rel- atively short time in the field. Don't be sur- prised at all if you have a very difficult time finding any supplier that can meet these cri- teria, but be surprised (and happy) if you find one with more shorts than opens in their products. really worth asking whether the supplier has successfully used HASL for BTC, BGA, or fine- pitch packages. Very few people can make that claim, and that was the reason for moving away from HASL in the first place. It was not too long ago that the predominant surface fin- ish was HASL. In addition to getting uniform solder coating with HASL, board warpage may also be an issue with HASL. For most com- panies using finer pitches, BGAs, and BTCs, HASL is really not an option. Because of availability constraints, many companies end up using both tin-lead and lead-free components on the same board. Such assemblies fall into what is commonly known as forward or backward compatibility scenar- ios. The key concern is the selection of the right peak reflow temperature since the tin-lead and lead-free components require very different peak reflow temperatures for proper reflow. Key questions to ask are as follows: • How does the assembler deal with tin-lead and lead-free BGAs on the same board? • What peak temperature is used to prevent overheating of tin-lead BGAs/QFPs with- out compromising the proper reflow of lead-free BGAs? • What reflow peak and TAL do you use when most of your components are tin-lead, but the BGAs are lead-free? • Does the supplier know the difference between TAL and true TAL? (TAL is time above liquids even if only one component sees that temperature. True TAL, on the