$)C
Guy,
This is an intuitive answer, rather than a scientific one or one from
experience, but first, some questions:
What problems are you having that you're even thinking of this as a
solution? Are you concerned about oxidisation or migration problems?
How are you going to achieve a 50 degree delta between one side of the
board and the other without some colossal heatsinking? Have a seriously
good thermal insulator in the middle of your board and refridgeration
plates rather than heating plates at the bottom of your reflow oven?
Do you propose to have the same delta between the bottom side and the
top side when reflowing the second side, or is this single sided?
If you try to have a large temperature delta between one side of the board
and the other, you will be tending to suck all the heat out of the hotter
side, which is presumably the side you are trying to reflow solder (heat
travels from hot regions to colder regions unless you have a good thermal
insulator between the two zones, such as a vacuum). There is no advantage
to this that I can see, as you will have great difficulty in achieving
reflow temperature, or, if you do achieve it, it may well be more difficult
to control since you'll be continually trying to combat a possibly varying
heat drain (thermal flow rates may vary through individual boards).
You will also wind up with bowed or convex boards, owing to differences in
degrees of expansion between the top layers and the bottom layers. This
will strain your solder joints when the board cools again, leading to
increased unreliability.
I can't think of many advantages, apart form reducing migration in double
sided boards by only reflowing each side once instead of twice.
Sorry to rain on you, but it's an intriguing question, and I would be
interested in knowing the background reason for asking it.
Regards
Pete Duncan
Asst Principal Engineer
ST Aerospace
Guy Ramsey
<gramsey@ACI- To: [log in to unmask]
CORP.ORG> cc:
Sent by: Subject: [TN] temperature delta on PWB
TechNet
<[log in to unmask]
ORG>
03/29/01
08:33 PM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
Guy Ramsey
Opinions / comments: What the cons are to restricting the secondary side of
a PCB from reflowing
during the topside reflow process? What problems are encountered when
keeping a 50 (, delta between bottom and top side of the PWB during the
reflow
process? What benefits might be encountered?
Guy Ramsey
American Competitiveness Institute
Senior Lab Technician / Instructor
610 362-1200 ext 107
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