Werner,
In the case of wave soldering, would not the concern mostly be with rise, as the transition from preheat to wave immersion is quite radical? After immersion, the outermost is exposed to ambient air (with much lower specific heat than solder) and the exterior-most of the part begins to cool at a more reasonable rate than it's recent rise, as the interior attempts to reach equilibrium, leveling the playing field, so to speak.
Regards,
Ed Popielarski
Sr. Mfg. Engr.
NBS Corporation
2950 Patrick Henry Dr.
Santa Clara, Ca. 95054
Ph: 408-654-1100
Fx: 408-654-1107
Cl: 408-234-1497
Cl: 949-581-6601
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Friday, February 11, 2011 1:07 PM
To: [log in to unmask]
Subject: Re: [TN] Wave soldering SMT capacitors
Hi Thomas,
What high rates of temperature change do, is set up transient internal thermal gradients causing differential thermal expansion [on heating] and contraction [on cooling]—and by the way, rapid cooling is worse than rapid heationg, because now you have the solidifying solder involved as well together with the interaction between warping components and PCBs.
Werner
-----Original Message-----
From: Gervascio, Thomas L <[log in to unmask]>
To: [log in to unmask]
Sent: Fri, Feb 11, 2011 3:58 pm
Subject: [TN] Wave soldering SMT capacitors
A friend of mine asked a good question, why is that most SMT capacitor specs
call out for a maximum heating rate at SMT of 2-4 C/sec but the same specs allow
a temperature delta at wave soldering of 100-125 C? In case of wave, the time
between preheat and soldering causes a heating rate in excess of the SMT heating
rates. But it is OK to subject the part to the wave solder but not SMT using
those heating and cooling rates?
I never could recall a logical explanation. Called several capacitor
manufacturer's and they seemed to dance around the question.
Any information would be helpful
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