Simply because of the soldermask-defined pads, the unit will start with
compromised reliability. Soldermask-defined pads are known to induce
voiding and have a propensity to crack at the edge of the ball
connection to the pad.
Adding a service cycle factor of 120 deg. C. will cause some major CTE
stresses on the pads directly tied to the power planes being used to
wick away the heat.
This combination could lead to a much lower time-to-failure.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ofer Cohen
Sent: Sunday, February 12, 2006 9:26 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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