HI,
What are the advantages and disadvantages to restricting the secondary side
of a double sided SMT PCB from reflowing during the primary side reflow
process?
I ask this question because there are two oven manufactures that claim they
can keep the secondary side of a PCB from reflowing during the primary side
reflow process.  Apparently they use nitrogen as a barrier.
The troublesome part of this is that when dealing with NiAu PCB's generally
your reflow temperatures are at the high end of the window (225 -230C) to
help wetting to the nickel barrier.  To keep this same temperature window
and trying to keep the secondary side from going into reflow would result in
the neighborhood of ~47C delta between top and bottom.
My gut feel is that there are no real benefits to doing this.  The problems
associated with inner layer shift,  mechanical stress on the joints,  and
all the other related problems to CTE mismatches tell me this is not a good
thing.  Any thoughts????



Shawn  C. Murphy
Process Engineer

6640 185th Ave NE
Redmond, WA 98052-5038
[log in to unmask] <mailto:[log in to unmask]>
Tel: (425) 885-0107 Ext. 678
Fax: (425)867-5600

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