The effect of the mechanical and physical properties of an underfill material on thermal
stress relief in a lead-free chip size package (CSP) solder joint has been investigated. Thin sheets of
underfill materials for the CSP solder joint were prepared and the mechanical and physical
properties of the sheets were investigated. Using these properties, thermal stress relief in an
encapsulated CSP lead-free solder joint with the underfill material was examined by a finite element
analysis method under thermal cycle conditions in the temperature range from 293K to 398K and
233K.
The effect of filler content in an underfill material on thermal fatigue life of a lead free solder joint of a chip size package (CSP) has been investigated. Three types of bisphenol F type epoxy based underfill material, which filler content was changed from 0 to 60 mass, were prepared. The thermal cycle test in a temperature range from -40 to 125°C was conducted to investigate thermal fatigue lives of solder joints encapsulated with underfill materials. Plastic strain range loaded in the solder joint under thermal cycle conditions was investigated by a finite element analysis method. From the thermal cycle test results, it was clarified that the thermal fatigue life of the solder joint increases with increasing filler content in the underfill material investigated. Moreover, a Coffin Manson type of relation was obtained between the plastic strain range and the thermal fatigue life. n value in the Coffin Manson's equation was estimated to be 1.8.
Accelerated thermal cycling (ATC) tests were conducted to investigate an effect of thermal
cycle conditions on thermal fatigue life of a chip size package (CSP) lead-free solder joint. A ternary
Sn-Ag-Cu alloy was used as a lead-free solder material. For frequency of thermal cycle (1~3 cycles/h)
and maximum (388~423 K) and minimum (223~273 K) temperatures investigated, the effects of
them on thermal fatigue life of the solder joint were slight. On the contrary, correlation was
recognized between temperature amplitude and thermal fatigue life of the solder joint. The thermal
fatigue life increased with decreasing temperature amplitude. The relationship obeyed the
Coffin-Manson’s type equation.
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