Characterising solder mask performace.

Although the electronics industry currently uses a number of test methods for evaluating solder masks, it has long recognised that they are inadequate, and that other (possibly new) methods are required. In this work a number of parameters of potential use in indicating the degree of cure of solder masks, have been evaluated. Tests using a series of measurement techniques have been undertaken on six different solder masks from three suppliers. Samples of each of the masks prepared under different cure conditions, were measured for thickness, surface roughness, hardness, surface reflectivity, contact angle of sessile drop, surface insulation resistance, capacitance and loss factor, and adhesion. In addition, process evaluations were undertaken for reflow soldering solder-balling and flux spread, as well as wave soldering solder-balling and skips. Underfill voiding measurements were also carried out. The results indicated that the majority of these measurements did not effectively, or consistently, indicate the degree of cure for the samples tested. Thickness affected many of the test methods to a much greater extent than did the degree of cure. The best correlation to degree of cure was found using surface insulation resistance testing, where the changes that occurred in SIR variables such as charge flow and SIR range were a good correlation for five of the six masks tested. This test method may have some use in determining degree of cure, but each material would need to be characterised individually and therefore the technique would be considered unsuitable as a test method for an end-user to apply across a range of boards from different suppliers. Additionally no distinct effects of UV cure could be detected with SIR testing or with other testing techniques. This project has also shown that the representative solder masks studied appear robust in relation to under or over curing. The materials also had very similar properties in terms of process performance. In addition the results indicated that solder masks may well cure to an equilibrium at room temperature faster than expected, which would be advantageous for stability of subsequent processing by assemblers.