Electrical and thermal bonds are such an integral part of electronic and semiconductor construction that they may often be taken for granted. Modern electronic assembly methods employ a myriad of bonding processes, each one a vital step in the manufacture of the final product. A typical consumer product such as a laptop computer may contain hundreds of thousands of bonds yet if one fails it will probably result in a system breakdown.
Bond strength measurement is far from the highest profile portion of the electronic and semiconductor industries but it has matured with it, in some cases unnoticed. This doesn’t alter the fact that a precise knowledge of bond strength quality measurement during product design and subsequent manufacture is directly related to product success, customer satisfaction and profitability.
To serve this need a modern bond test system must be capable of accurately testing bond wires, solder bumps, dies, leads, chips, lids as well as other applications with strengths varying from a few grams of force to as much as hundreds of kilograms of force. Investigating the roots of bond testing, outlining what is required to perform a good bond test and what a modern bond tester should be capable of performing is essential in defining an accurate bond testing protocol.
Getting back to basics, why measure bond strength in the first place? Bonds fail either in production or end use and these failures can be caused by their geometry, material or processing factors. These factors greatly affect and ultimately define the quality and integrity of the bond. Measuring bond strength is then a useful tool during the design process and for quality control to minimize product variability, ensure manufacturing yield and increase end product reliability.
Bond testing has been around for so long you might not have thought what it is really for and how it should be done? The basic types of bond testing include pull and shear which are applied to either the destruction of the bond or to some force below this value in the form of a non destructive test. A push test is sometimes used but this is closely related to the pull test.
Non destructive tests are normally used when extreme reliability is required such as in military or aerospace applications. In these cases, every bond is tested to ensure the utmost quality level making this routine relatively time consuming and costly. More commonly bonds are tested to destruction on a batch sampling basis with the peak force and failure mode being recorded for statistical analysis. This is indeed a good starting point but requires examination in more detail. The best place to start is with the failure mode. This is based on the assumption that there is a particular failure mode, or range of modes, that may occur within a product. It is therefore reasonable to assume that the bond test should replicate the mode, or modes of interest. However, exact replication is not always possible. The test load must be applied to some part of the sample and transferred through the sample to the bond. If this part of the sample is the only option and is weaker than the bond itself, the sample will fail before the bond.