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62 SMT007 MAGAZINE I OCTOBER 2019 been mostly replaced by BTCs and gull-wing devices. Today's VPS has also improved since most of them have built-in preheating systems. Despite these improvements, you will be hard- pressed to find significant VPS users. The con- vection system provides efficient and uniform heating without the inherent problems of VPS and is the most common process for reflow soldering. Soldering Options for Backward Compatibility With the widespread use of lead-free solder- ing, companies must rethink their soldering options, especially when dealing with back- ward and forward compatibility. Backward compatibility is a situation in which the major- ity of components are tin-lead, but some are lead-free. Forward compatibility is a reverse— the majority of components are lead-free, but some are tin-lead. Forward compatibil- ity is rarely an issue since it rarely comes up, but backward compatibility is a very common situation. Backward compatibility is mostly a problem in military and space applications since this industry has not fully adopted lead-free, but they are stuck with lead-free components since component suppliers don't find it cost-effec- tive to sell both tin-lead and lead-free com- ponents. All lead-free components can be sol- dered using tin-lead solder paste and a tin-lead reflow profile except lead-free BGAs. Some companies replace lead-free BGA balls with tin-lead balls at considerable expense and sol- der the reballed BGAs using a tin-lead process. Meanwhile, others use a profile with a lower peak temperature than the lead-free profile but a higher peak temperature than the tin-lead profile—essentially, a profile that is a compro- mise between a tin-lead and lead-free profile. In either situation, there is the problem of soldering components with differing heat-input requirements, and there are serious trade- offs. You cannot go to a higher temperature to reflow just a few lead-free BGAs, as the major- ity of tin-lead components will likely be dam- aged. And you cannot stay with the tin-lead profile because lead-free BGA balls will not fully melt, and the balls do not collapse, which is a key requirement for improved BGA solder joint reliability. I will cover this complicated subject in detail in future columns. Selective Soldering Options for Mixed Assemblies Mixed-assembly boards—those contain- ing both surface-mount technology (SMT) and through-hole components—represent the majority of products in our industry. What do you do when you have a mixed assembly with SMT and through-hole components on the same board? Here are some commonly used selective soldering options to consider. 1. The use of non-metallic fixtures is a common method to solder through-hole components selectively on a mixed- assembly board. However, this method only works when the board is designed correctly. Otherwise, the fixture requires several iterations to come up with the final fixture that will expose the through- hole components and completely hide surface-mount components on the bottom side. This option can be very expensive, depending on the mix of products you must deal with, and requires considerable storage space for fixtures. 2. Another method generally referred to as solder fountains uses a metallic fixture that covers the solder pot. The solder comes out like a fountain at the designated locations under the through- hole leads. These fixtures can also be very

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