Package attachment to thermal base by means of soldering

AN3232

20/26

Doc ID 17594 Rev 3

10

Package attachment to thermal base by means of

soldering

10.1

Hardware

●

Printed circuit or wiring board (PCB or PWB)

●

Solderable core layer, normally copper

●

STAC package (can be flangeless or boltdown version)

●

Pb or Pb-free Solder preform or similar solder paste

●

Solder reflow equipment, such as an oven

●

Optional mechanical hardware to locate the package

10.2

Solder reflow equipment and methods

There are many methods for soldering; which method to use is based on factors outside the

scope of this application note. For the STAC package comprising a material set in use for

decades, ST feels comfortable recommending a convection reflow oven, preferably with 5

zones plus a cooling strategy. For example, the SEHO FDS “MAXIPOWER” unit depicted in

Figure 9, with independent top and bottom heaters and a refrigerated cooling zone, has

been used in testing STAC packages for reflow soldering and is well-known throughout the

industry.

Figure 9.

SEHO FDS “MAXIPOWER” reflow oven

An important consideration for reflowing power packages like the STAC is that the

temperature profile must be adjusted on a case-by-case application basis. No universal

profile can account for the large variation in thermal mass between various RF Power

Amplifier (RFPA) assemblies. An RFPA might consist of a relatively small PCB and thin core

layer, which can be soldered with a relatively fast conveyor speed. On the other hand, a

large area RFPA pallet, having a thick copper core layer, requires much slower movement

through the reflow oven to achieve good soldering. The PCB acts in insulating the top of the

core layer from topside heating, therefore a programming balance between top and bottom