Patent Application
20080307643
This invention relates to methods of assembling discrete power packages and a heat sink on a printed circuit board.
Conventional processes for physically and electrically connecting a plurality of leaded power packages to a printed circuit board have involved applying plastic standoff spacers to the printed circuit board, inserting the power packages onto the printed circuit board with the leads extending through lead holes provided in the printed circuit board, holding the properly positioned package in place on the printed circuit board with a suitable clamping fixture, and wave soldering the leads to the underside of the printed circuit board. Thereafter, the assembly comprising the power packages wave soldered to the printed circuit board is detached from the clamping fixture and attached to a heat sink or rail. Intimate contact or minimal space between the power package surface and the heat rail is desired. However, the typical assembly process creates differences in height among the power packages on the printed circuit board. The typical assembly process also causes individual power packages to be tilted with respect to each other at different angles.
To achieve intimate contact across the entire package area using known techniques, the package leads must be bent so that the outer surfaces of the discrete power packages are all at the same height and located in substantially the same plane. Typically, a clamping mechanism is also needed to hold the power device in contact with the heat rail due to the resilient memory (i.e., the tendency for a metal to return or spring back toward its original shape after deformation) of the bent leads. Using the circuit board as the reference plane for positioning of the power devices in accordance with conventional processes causes the amount of lead bending to be equal to the sum of tolerances (i.e., dimensional variations) for circuit board warpage, lead forming, power device thickness and heat sink flatness. This bending, which is typically greater than 0.01 inches in magnitude, can cause physical damage to the power package resulting in reduced quality and reduced reliability of the assembly.
The process of the invention eliminates, or at least substantially reduces, bending of leads during assembly of power packages and a thermal heat sink or rail on a printed circuit board. Bending is eliminated or at least substantially reduced by attaching the power packages to the heat rail prior to attaching the leads to the printed circuit board. After the power packages are attached to the thermal heat sink or rail, the leads are inserted through the appropriate lead holes in the circuit board and are allowed to move freely without bending during a subsequent soldering step. Because intimate contact between the upper or outer exposed surfaces of the power packages and the thermal heat sink or rail is achieved or perfected prior to soldering of the leads of the power packages to the printed circuit board, and because the leads move freely through the appropriate lead holes in the printed circuit board during the soldering process, final assembly can be achieved without significant bending of the leads.
Using the heat sink as a reference plane for positioning of the power devices in accordance with this invention eliminates lead bending contributions attributable to tolerances (i.e., dimensional variations) for board warpage, lead forming, power device thickness and heat sink flatness. The processes of this invention allow the solder joint to compensate for these variations.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
The steps in a conventional process for assembling power packages and a thermal heat rail on a printed circuit board is illustrated schematically in
Unfortunately, the process described above and shown in
A process in accordance with the invention is illustrated schematically in
With excellent planar heat rail surface to planar power package surface contact being provided while the leads are still loosely inserted in the printed circuit board, the subassembly is transferred, in step 160, to a wave solder pallet and the leads of the power packages are wave soldered in step 170 to the printed circuit board. Finally, in step 180, the completed assembly is removed from a fixture used during the soldering step.
A suitable fixture for preparing a subassembly in which the power packages 200 are attached to a heat rail 204 is shown in
As an alternative to adhesively affixing an upper surface of the power packages to the printed circuit board, the power packages may be temporarily affixed to the heat sink using a mechanical clamp mechanism. After soldering is complete, the power devices may be held in intimate contact with the heat sink using a spring fixture similar to that described above and illustrated in
Before and during assembly of the fixture (205, 207 and 208) around the subassembly, power packages 200 are loosely held on printed circuit board 202. A force may be applied to the underside of the power packages using springs 210 that act through plungers or pins 209 which extend upwardly from spring 210 and through a plunger aperture provided through printed circuit board 202. The spring 210 may be an elastomeric member, a metal coil spring, a metal leaf spring or any other suitable means for urging power packages 200 against heat rail or heat sink 204. The fixture is used to achieve intimate planar surface to planar surface contact between the power packages 200 and the heat sink 204.
After the power packages have been attached to the heat sink, the same fixture may be used for wave soldering the leads 201 of the power packages 200 as shown in
As an alternative to using the upper fixture to hold the heat sink and power devices in place during soldering, a rigid heat sink may itself be used as a fixturing device as shown in
Employing a process of this invention using an adhesive to bond the power packages 200 to the thermal heat sink 204, it was found that very uniform bondline thicknesses (the thickness of the adhesive layer between the power packages and the thermal heat sink) could be achieved.
It will be understood by those who practice the invention and those skilled in the art that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.
