We have used silver filled epoxy applied through a .032" hole in the board
after the component was mounted and soldered. The material was very
compliant and was approx. 60% silver. There were pre-cleaning, adhesion and
curing issues, but then, our primary concern was electrical conduction, not
thermal. Also used on some products is a highly thermal conductive silicone
adhesive, also applied and cured after component mounting.
R. G. Blomberg
Honeywell
Staff Production Engineer
-----Original Message-----
From: Tucker, Steve (KS) [mailto:[log in to unmask]]
Sent: Thursday, August 26, 1999 9:50 AM
To: [log in to unmask]
Subject: [TN] Thermal bonding to PCB's
Technetters,
I serve as the manufacturing rep. for a design team and am faced with a
question regarding the best method for heat sinking a processor on a circuit
board. Any suggestions or comments would be greatly appreciated.
DETAILS
This component is an Intel SB80486DX2SX50, a 208 lead, 20 mil pitch, SQFP
package. This component is expected to generate ~1.2W of energy in our
application. The PCB is 14 layer, HASL finished, and approximately 4"X6".
The area directly under the body of this component is a HASL finished ground
plane approximately .9" Square designed to serve as a heat sink for this
part. This plane is internally connected to the other PCB grounds.The
product is subjected to 20G's vibration at temperatures ranging from -40C to
+80C.
BACKGROUND
Most members of the team want this component to be heat sinked from the
bottom via thermal contact with the PCB ground plane rather than applying a
heat sink to the top of the component and relying on air flow to dissipate
heat. Various methods have been suggested, none of which I am thrilled with.
My concern is that any under component heat sink material I apply would have
to allow the component to be placed correctly and allow the part to seat
properly during reflow. This part, by the way, spec's a dimension from the
bottom of its body to the bottom of leads of .13mm +.12mm -.08mm. With this
variation in mind I'm stuck for a good answer.
My question is: what is the best method to provide a good thermal bond under
this component while still allowing machine placement and without causing
additional solderability or placement problems.
Regards, Steve Tucker
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