INTERMETALLIC COMPOUNDS GROWTH ANALYSIS WITH TWO METHODS: X-RAY DIFFRACTION COMPARED TO THE COMBINATION OF X-RAY FLUORESCENCE (XRF) AND COULOMETRIC STRIPPING (CS)

摘要

This article presents an intermetallic compounds (IMC) growth study, specifically tin-copper intermetallic (Sn-Cu) in Immersion Tin PCB's. Its impact on reliability is well known in the electronics industry because if IMC grows too thick, the electronic joints become brittle and can be easily broken or if IMC reaches the surface can be easily oxidized. In this case, we are interested in knowing its growth under different temperature conditions, but we are using one different method to measure the growth and compared to another published before. The selected method was X-Ray Diffraction (XRD). XRD has the advantage that is not destructive and can take measurements in a relative short time. The intensity of the crystal peak is correlated with its relative concentration. The method to compare was the one including the combination of X-ray Fluorescence (XRF) and Coulometric Stripping (CS). Immersion Tin is the material to study. The second method (IMC and CS) showed measures the thickness of the Cu diffusion into tin. The results show the IMC starts growing is in the form of dispersed granules oriented in different spatial planes; however, when they reach a certain height (approximately 0.5 um), they merge and are oriented towards the same diffraction plane; this plane is detectable by means of XRD; before that, no clear growth is detected due to the different crystal orientations . When the crystals are oriented, there is a high correlation between both methods in IMC thickness measurements. XRD could also detect how Sn element diminish its presence if IMC growths in that case. Other elements or compounds such as SnO, Ag3 SN and Ag kept relative stable in the same concentration. XRD was therefore tested as an alternative measurement method of IMC thickness in case some conditions are met.