Hi Tom & David,
To the question:
>>2. Should I take measurements with the first loaded board w/
>>solderpaste? If so, where should I place the thermocouples?
there is a more practical, common-sense answer than relying on
>a mix of experience and judgement for determining thermocouple locations
or to
>a) Build a computational model of the the board during the process.
>This is time consuming and expensive, but we have shown that it can
>be fairly accurate in predicting what will happen. You can also check/
>design your reflow profile using the model or do "what if?" design
>studies on e.g. component locations using the model.
>b) Buy a reflow oven with an IR scanner in it, which will let you
>verify the peak temperature of every component on the board. This
>also lets you check every board hits the right temperatures.
Obviously, Tom is lacking the experience and judgement necessary, otherwise he
would not have asked the question, and a) and b) or both too resource
intensive and in addition not necessary.
Consider, what it is you are trying to accomplish, assuming you have a decent
reflow machine that gives good uniformity of heat transfer. Primarily, you
want to assure that the slowest heating solder joint reaches the Liquidus
temperature for the solder used plus 20C for copper leads (plus 30 to 35C for
nickel surfaces or Alloy 42) for at least 3 seconds to assure good wetting.
Secondarily, you want to make sure, that you are not above Liquidus too long
(90 to 100 seconds) to keep IMC layer thickness down (this is much less
important than assuring wetting) and that the peak temperatures reached by
some of the smaller components is not too high for them.
Thus, the primary locations for a thermocouple should be a solder joint in the
center of one side of the largest componet (largest thermal mass) and a solder
joint in the middle underneath the largest BGA. Secondarily, choose a
thermocouple location on the most heat sensitive component (connector).
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]
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