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What are the characteristics of a good solder joint? Generally, from a visual standpoint, they should be smooth, bright, shiny, clean, and have a nice concave solder fillet. At least, these characteristics describe solder joints for through-hole components. What about for surface-mount devices? Or bottom terminated components? Moreover, how do you ensure that inside those joints, good intermetallic bonds are formed?
Since all solder joints aren’t created equal, the first step is to understand what is expected or required of that joint, according to Rick Short of Indium Corp., during my interview with him at the recent NEPCON South China trade exhibition in Shenzhen, China.
Solder joints play several different roles, and they represent several different opportunities to be a problem, a partial improvement, or a complete improvement. For example, there are solder joints that are necessary for physical strengths, such as holding components, like connectors. We often wiggle and yank our phone jacks—there’s a little bit of solder trying to hold all that together, and we keep doing it over the course of ownership of the device. So, those joints must be very robust with regard to physical strength. Other solder joints are merely there to conduct electricity. They don’t have much of a harsh life at all, and it is a relatively easy life for them. Other solder joints, meanwhile, are involved in very high heat dissipation demands. They must conduct electricity and heat, and they might also need to address physical strength issues. So, many different opportunities exist for solder joints to either cause you problems or contribute to the success of your finished goods.
Nobody, including myself, wants field failure. I often travel overseas for work, so I definitely don’t want the airplane I’m on to experience that "field failure." But going back to our topic on solder joints, I vividly recall that fatal plane crash a few years ago that was mainly caused by cracked solder joints in the subassembly unit that controls the rudder. The crash was tragic, but the cause of it is also the reality. I am not sure about the other technical details on the plane, but it had already logged 23,039 flight hours since its manufacture, and 13,610 cycles (an aircraft cycle means takeoff and landing).
Just imagine the many hours that these PCB assemblies have been subjected to harsh conditions—component breakdowns are inevitable. Which is why the reliability of PCB assemblies remains very critical.
In our recent survey on soldering, we identified many challenges to address during the soldering process to ensure good solder joints. These include solder paste selection, thermal issues and reflow profiles, voiding, and component size variations, to name a few.
To read the full version of this article, which appeared in the October 2017 issue of SMT Magazine, click here.