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Thermal Profiling and Vapor Phase Soldering

Posted on Sep 09, 2010

There has been some new talk by some of the best quality conscious electronic manufactures about the many benefits of an older soldering process: vapor phase soldering. Vapor phase soldering has a lot of good things to offer, now that we have gotten past the stigma of the old CFC fluids and moved on to newer chemistries. The maximum temperature that the assembly can be subjected to is dictated by the boiling point of the fluid being vaporized. Because the boiling point of the fluid is a physical constant, you might think, “Why bother running a thermal profile on the assembly being soldered.” This idea should be considered carefully, and here are some reasons why thermal profiling in vapor phase soldering is still a very good idea:
  1. Although the boiling point of the vapor phase fluids is a physical attribute that limits the maximum temperature, the condensation of the fluid onto the components can impart a lot of heat, real fast. This can subject components to the old thermal shock problem, and unless this heating rate is carefully controlled by the vapor phase machine, you may well be shocking the components. Thermal profiling is the only way to show this is under control.
  2. The maximum temperature is a function of the fluid type, and one needs to be sure the correct fluid is being use. There is a fluid whose boiling point is hot enough for lead free soldering, and not too hot for leaded soldering, about 230ºC. This "happy medium" is a good compromise, so one does not have to own two different vapor phase machines, or change fluids from one process to the other, but it is another reason why thermal profiling is a good idea: to prove that the process is meeting the need of the solder paste and the limits of the components.
  3. A process undocumented is a process out of control. Unless you have some evidence that the thermal profile is meeting the requirement of the solder paste and the limits of the components, you cannot prove the process is in control statistically. You can't make process control charts if you don't measure the process. This is at the heart of a good Thermal Quality Management (ThQM) program.
  4. Your customer still wants to know what the thermal profile looks like. No matter how you solder your customer’s boards, they still want to know what they were subjected to, thermally. This is your assurance to them that you have treated their product properly.
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