Study of the interface microstructure of Sn-Ag-Cu lead-free solders\nand the effect of solder volume on intermetallic layer formation

  • Salam B
  • Ekere N
  • Rajkumar D
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There are a number of issues regarding the use of Sn-Ag-Cu alloys,
including the solderability and long-term reliability of the solder
joints, which require further study. The lower solderability of Sn-Ag-Cu
solder can alter the interface and microstructure of the solder joint
formed because of the differing reaction rates between the molten solder
and substrate surface. This also has an impact on the nature and extent
of the intermetallic compounds formed at the interface, as the
intermetallic is generally more brittle than the base metal. This can
negatively impact the solder joint reliability. In this paper we report
a study on the effect of solder volume on intermetallic layer formation
and thickness. For lead-free soldering this could prove to be very
important, as a wide range of devices and components of varying joint
size, e.g. plastic quad flat pack (PQFP), ball grid array (BGA),
chip-scale packaging (CSP), and flip chip, may need to be assembled on a
typical board. This means that the nature and thickness of the
intermetallic layer formed for each joint size will be different. In the
study, solder joints of different sizes representing different devices
were used for evaluating the effect of solder volume on intermetallic
compound formation. The layer thickness and microstructure were analyzed
using scanning electron microscopy (SEM). SEM analysis was also carried
out on joint micro-sections, which has undergone temperature cycling to
evaluate the effect of intermetallic layer the joint reliability. Our
results show that increasing the solder volume (and solder joint size)
does not significantly affect the growth of the intermetallic layer
thickness. Therefore the intermetallic layer thickness provides the
lower limit for solder joint design for ultra-fine pitch flip-chip

Author-supplied keywords

  • and solder joint reliability
  • intermetallic compound formation
  • lead-free soldering
  • tin-silver-copper alloys

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  • B. Salam

  • N.N. Ekere

  • D. Rajkumar

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