1.2 Attachment of Package to Substrate

The attachment of the IC to the substrate itself can be accomplished with the most standard epoxies or adhesives, depending on the mechanical and environmental requirements of the system where it is used. The adhesive must be applied in a pattern that spreads evenly along the package surface and does not leave large voids. The figure below shows an approximate example.

Figure 1-3. Adhesive Attachment

The amount of adhesive deposited depends upon the properties of the adhesive used, and the relevant application notes of the adhesive supplier must be referenced. To provide even coverage of the rear surface of the package to a uniform depth of 0.2 mm requires the application of ~3 mm3 of adhesive. Proper deposition of adhesive may produce even distribution beneath the package with no voids, as illustrated below:

Figure 1-4. Correct – Even Adhesive Coverage

If the deposition profile/pattern does not produce an even spread of adhesive, voids in the adhesive can result in stress on the package when under pressure with the compression connector. The following figure illustrates an example:

Figure 1-5. Incorrect – Uneven Adhesive Coverage Causing Voids

Adhesive Selection

The choice of an adhesive is dependent upon many factors including the materials to be bonded, the environmental conditions which the assembly will be stored and operated in, environmental regulations and the actual characteristics of the adhesive. For this reason, a general recommendation of what adhesive can be used for a given application cannot be made. See Appendix B - Adhesives for an example of an adhesive that may be acceptable for this application.