Real-time studies of tin surface and grain structure show where whiskers initiate

Eric Chason, Brown University, DMR 0856229

Tin whiskers are a critical issue affecting the reliability of electronic systems. Growing spontaneously out of tin coatings, they cause short circuits and system failures. To understand why they form, we need to know what is special about the sites that they grow from. We use scanning electron microscopy (SEM) to monitor the surface morphology (figs. a and c) and electron backscattering diffraction (EBSD, fig. b) to map the corresponding grain orientation of the whiskering grains (circled regions). This required developing a special technique (called “peel-off”) to grow ultra-flat surfaces that allowed EBSD to be used. The dots in the inverse pole figure show the orientations of the grains that form into whiskers, indicating that they tend to grow out of grains with an (010) orientation surrounded by (001) grains.

Tin surface and grain structure

a) as-deposited

c) after 65 hrs

30 m

(001)

(011)

b) grain orientation

d) (010)

Images of a),c) tin surface, b) grain orientation and d) inverse pole figure

Research to support lead-free manufacturing

Eric Chason, Brown University, DMR 0856229

Lead and other hazardous substances have been removed from electronics manufacturing for environmental reasons. Tin coatings without lead (lead-free) have a problem with whiskers that grow out of the surface and cause short circuits. We wrote a chapter about tin whiskers for this book on lead-free solders to educate industrial users about the causes and mechanisms of whisker growth. We have also presented our research in invited presentations at the annual lead-free solder symposium of the TMS meeting and the weekly industrial whisker telecon.