Military and Aerospace companies need the extra performance of SnPb coated components.

Fischer's new X-ray software helps simplify the process.

For many years the replacement of leaded solder with alternative alloys or pure metals has been a constant issue for "high reliability" applications, including the aerospace and military sectors, where a minimum proportion of lead is still required.  Lead is demanded in these applications due to the unfortunate and uncontrollable property of pure tin to grow fine hair like whiskers out from solder joints.  These can span quite large distances to connect with other joints and cause short circuits. They can be clearly seen in Fig. 1. Here tin whiskers can literally be fatal and tin-lead alloys must be used as the lead prevents this unfortunate metallurgical property of pure tin.  Lead is now banned in most commercial products, so the growth of whiskers will inevitably increase.

Fig.1 The images show a tin whisker, which has spontaneously grown between two pure tin ic pads, creating a short circuit

The identification of new solder material, both in bulk form and on manufactured circuit boards is an important application for producers and users and the advances in X-ray analysis software by Fischer have simplified this task.  The new software uses a function called Class of Materials, which groups sets of specific applications in a library and when a sample is presented to the instrument this library is scanned and the application with the closest fit selected.  Fig. 2 shows a range of complex solders but clear distinctions can be seen in the different coloured spectra.

Fig.2  The green spectrum is pure tin, while the red is of SAC305 (SnCu0.5Ag3).  The yellow is SnPb3, used in high reliability soldering.  The blue is from a SnBi solder with 1% Bi.  Clear differences can be seen by the small peaks of the accompanying elements Cu, Ag, Pb and Bi being clearly visible.

The analysis is a very complex mathematical task as elemental radiation is affected by other components in the alloy and in the case of coatings the underlayer radiation is partially absorbed by layers on top.  The thickness of each layer also complicates the situation, as the model must accommodate a whole range of various layer thicknesses.  Fig. 3 shows the analysis of thin solder coatings; bromine in the PCB material can now be compensated to provide better accuracy.

Fig 3. Thin solder coatings (pure tin,SnBi and SnPb3) on PCB material, in the presence of Cu-brominated epoxy PCB.

Outlook

Analysis of samples has always required a certain knowledge of what to expect before accurate quantitative results could be obtained.  This may have involved studying the spectrum before setting up an application for the precise measurement.  The automatic Class determination removes this step and any possible operator errors.

Phone 01590 684100 to discuss your special application or email xray@fischergb.co.uk