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Copper pillars can be tested in multiple ways. The best method depends on your failure mode of interest and the construction of your sample. If you are interested in the interconnect between the solder and the copper, it depends on the shape of the construction whether a Cold Bump Pull (CBP) type test is feasible. If not, a shear may be the only effective test to qualify your process.
The test sequence for the CBP-approach is as follows:
Whether to use a closing distance or constant closing force on the tweezer jaws depends on which method produces the most failure modes of interest or highest force. The failure modes are;
Copper to substrate breaks are a common failure mode. When this happens the bond strength and failure mode of the copper to the substrate pad is known. Copper solder break is not so common and probably indicates a non-wet. Copper extrusions indicate strong bonds.
The test sequence for the shear-approach is as follows:
The rationale behind the top landing and the rest of this test sequence is further explained on our copper pillar test type page. In this how-to we focus on tweezer pull tests only.
The failure mode of interest between the copper and the pad can be produced by either a pull or a shear test and the measured bond strength used for your process control. In many other applications pull testing is typically preferred because the bond is subjected to a simple tensile load, distributed over the bond area. The bond separation is clean making failure mode analysis of the surfaces relatively easy. Unlike solder bumps, copper is relatively hard and gripping it therefore easier.
Solder balls require precise reforming in order to be able to apply a meaningful test load on to the bond. Copper also has to be reformed in order to be able to grip it but this takes the form of well know gripping methods such as plain surfaces and friction, a few serrations that slightly reform the copper in order to get a mechanical grip or a slight tapper.
An alternative to pull testing is shear testing. We tend to prefer shear testing to pull testing for copper pillar.