Issue link: https://iconnect007.uberflip.com/i/881969
26 SMT Magazine • October 2017 ACHIEVING THE PERFECT SOLDER JOINT: THE MANY PERSPECTIVES ON SOLDERING effort, and it just wasn't happening. So that's the kind of design issue that we're dealing with. The tools are fantastic for what they do, but the thermal mass of this board is so heavy that there's just nothing we can do with it." To address the issue, Burns says they used a selective soldering system. "With the selective solder system, you have a lot more advantage be- cause you have the heat of the solder pot plus you can push the solder through the hole. That was our solution in this case. But if we have to re- work those, we'll have huge problems. The com- ponents that are on this don't allow us to use a pre-heat system just because of the design of the component. Flipping the board over and doing the bottom side just is not effective," he explains. Ensuring Solder Joint Reliability Sometimes, the reliability of the solder joints is what stands between life and death. In a plane crash a few years ago, the investigators found out after a year of investigation that the main contributor of the crash were cracked sol- der joints on a PCB of the rudder travel limiter unit. The cracked solder joints resulted in a loss of electrical continuity to the rudder system, leading to its failure, and the fatal plane crash. "I'm familiar with that case," says Roush. "As a point of fact, I have pictures of it and pictures of the solder joint, and the example I'm look- ing at looks like it's a through-hole component and you can see the joint, but if you look at the actual solder fillet that's supposed to have been formed, it's not formed correctly. So, it looks like improper assembly and then improper inspec- tion. They didn't solder it right and then they didn't catch it in inspection. A combination of things happened. It's a poor-looking solder joint and I don't think anyone who's done any length of soldering would have found that acceptable. So, it just goes to show that keeping to the basics a lot of times of forming a good solder joint are what can solve the problems." "I think it's critical to choose a material that fits your application and then process it prop- erly, which seems like a no-brainer, but the inspection is also critical," says Sandy-Smith. "I agree with all the comments I've just heard," notes Burns. "From our perspective, ide- ally the automated processes are going to work properly; we select the right materials, we select the process, we select the profiles, we run every- thing and it works fine. Then we do inspect to a point depending on what the product is. If it's aviation, anything aerospace or anything crit- ical, typically we have 100% visual inspection at that point. A lot of the commercial products, not so much because if you inspect, you have a lot of costs and a lot of time, but any of the critical systems are inspected 100%. The other end of it is training of the operators that are do- ing the hand soldering. You have to have peo- ple that are trained to solder correctly and to recognize what a good and a bad solder connec- tion really is, including your inspectors. So, if your operators are all trained properly and your systems are working properly, that type of thing should be an extreme rarity." SMT Electronic module of the rudder travel limiter unit. (Source: National Transportation Safety Committee, Indonesia)