More Thermal Mass Issues

Yesterday I wrote about some thermal mass related traps. Here’s another one we see now and then.

The top image shows good flat-topped caps. The bottom and inset has overheated bulged and damaged caps. These caps are RoHS compliant — supposedly. Their data sheet calls them out as RoHS compliant and their temperature specs and recommended reflow profiles indicate RoHS compliance. So what happened?

Well, they are compliant pretty much only in singles. A single of these caps will solder up fine and not be damaged. However, put four in close proximity like this and the solder paste on the inside pads will not melt at the recommended profile. They need a bit more heat because the thermal mass of the four parts close together sinks heat away from the inside solder pads. In the end, we hand soldered these specific parts to solve the problem, but for production, either a more thermally robust part would be needed or the part spacing would need to be changed to compensate for the combined thermal mass.

Duane Benson
“Hot” is a relative term

2 thoughts on “More Thermal Mass Issues

  1. Well… you could (might?) fix this with more space between components…
    You could work more closely with the process engineer involved…

    I have
    – designed re-flow furnaces (controls) for many years ..
    – also responsible for setting the profiles on my designs in production.
    – have performed profile optimizing / analysis for many thermal processes (thick film hybrids/smt/etc..)

    To fix the problem with these caps.. may involve:
    – longer duration at peak .. (with lower peak)
    – different furnace design … how much IR vs forced convection? .. the differences in cheap vs expensive furnace design often shows up on tough soldering jobs (like extremes in mass).
    – Atmosphere (nitrogen).. will affect the wetting.. affecting required profile.
    – solder paste flux ..
    – solder …
    – etc…

    As you are pointing out….
    It is much more complicated than just placing and routing to meet elec/mech requirements.

    a good pcb layout designer .. knows the limits of the production line also.
    (besides knowing something about circuit design/physics/mechanical/etc..)

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