Strain Gauge Measurements on Printed Circuit Boards
With the introduction and obligatory application of lead-free solders the manufacturers of electronic componentry were suddenly challenged with a whole set of totally new problems. That applies in particular to the automotive sector. Unarguably, for environmental protection reasons the introduction of such solders was an absolute necessity. It seems, though, that one hasn’t thought of the significant differences in the mechanical properties of leaded and lead-free solders.
Leaded solders are relatively soft, and they are highly deformable under mechanical load. They are functionally highly reliable, even under high mechanical load. Lead-free solders, however, are substantially harder and will virtually unobstructed transfer such mechanical loads into the electronic components on the boards.
That particularly endangers ceramic capacitors as well as BGA-package chips. The latter introduce a significant mechanical enforcement into the PC-board due to the high number of solder dots. Leaded solders would partly absorb the board deformation, but with lead-free solders that would not be the case, and cracks in the solder joints may occur, particularly in edges. BGA packages may even break under such circumstances.
A further problem is the thermal mismatch with complete instrumentation sets in automobile industry. Aluminium or plastic housings are regularly used here with thermal coefficients of expansion of 23 ppm/K. The coefficient of the PC-board material is normally 16 ppm/K, and that of the electronic components is often lower than 10 ppm/K, and there is no way for lead-free solder material to absorb stresses generated by such high thermal mismatch of different materials.
In order to solve such problems design alterations have to be introduced. Their efficiency, however, has to be experimentally verified. In cooperation with customers, especially with Robert Bosch GmbH, ESA Messtechnik GmbH has started right from the beginning to develop strain gauge measurement techniques and apply them in industrial practise. We were intensively involved in developing the currently valid strain limits standards for PC-boards in the automotive sector, and have accumulated a host of experience in strain gauge measurements on PC-boards. Moreover, we are the experts when it comes to corrections of thermal influences on strain gauges themselves which is highly important when temperature change tests are to be performed. Other fields of expertise are measurements during sequential production steps as well as drop tests. Our test equipment are comprising of highly specialised state-of-the art instrumentation.
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