How Does the Right Preform Size Design Affect Yield?

Custom Preform Sizes and Yield Risk

Having the right preform size affects yield in a big way.  Preform suppliers offer custom size preforms, and with existing in-house tooling may offer previously designed preforms that customers can use to avoid the cost of custom tooling.  In some cases this can be quite acceptable, but in certain cases this can be a disaster and greatly impact yield. Example:  A customer provides a preform size with a center hole for a device assembly process. In figure 1 below you see the preform is undersized compared to the available area. When pushed into a corner, the hole of the preform is not concentric with the hole in the underlying part.   

Undersized Preforms Can Block Holes

With an undersized preform there is a possibility that the preform can move during the placement of the device and could partially block the hole in the underlying part as shown. The hole must remain free of solder for the device to function properly. 

Using a Larger Preform or Hole

The solution to this can be accomplished by obtaining a larger preform and/or a larger hole, as in Figure 2.

The results of this are shown on the right side of Figure 3. On the left is a part with a hole plugged with solder, the result of soldering that which is shown in Figure 1.


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Zap (Pierino) Zappella
Process Development Engineer
SST International