Ofer,
I have to agree with the others on the pad thing here, but I can add
to the heat idea perhaps. We use a 2oz copper plane as the first layer
under the surface layer to remove heat from the board/parts. In space
you have to remove heat by conduction since air is not an option. :-)
Anyway, the copper planes are the major way to remove the heat if a heat
sink won't fit or is too small. (I never have to worry about those heat
sinks. :-) ) From what our mechanical engineer has said, if you have a
.0035" dielectric and mechanically fasten the edges of the board, (the
planes are Chassis ground in my case) to a larger heat sink, it works
pretty darn good. Now the hitch in the get-a-long is the controlled
impedance, thickness, and the use of polyimide and such. I'm using
polyimide /41 materials and heavy copper planes, I'm not sure if that's
and option for you? $$$ I'm not sure if this helps or not, but it's my
$.02 worth... BTW, on my BGA's so far I'm still using a dog bone pattern
and haven't used VIP, YET.
Later...
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ofer Cohen
Sent: Sunday, February 12, 2006 8:26 AM
To: [log in to unmask]
Subject: [TN] Heat storage within PCB
Hi all,
One of the designers came with an idea: he has a high power component.
The space he has for a heat sink is small, so he wants to dissipate the
heat through the power layers. The component is a BGA, so the idea is to
put it on a metallic-capped via-in-pad by designing pad diameter of 24
mils, then define the solder area to 18 mils by solder mask.
Two questions:
1. Provided the temperature may rise to 120 degC, and the laminate
is high Tg low cost FR4 - are there any potential risks to the long term
reliability?
2. Are there any restrictions/hazards on deployment of defined
pads?
Regards
Ofer Cohen
Manager - Quality Assurance and Reliability
SIEMENS COM FN A SB
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