2 research outputs found
Study of self-alignment of μBGA packages
In this paper, a detailed study of the self-alignment
of BGA packages is presented. Complete self-alignment can be
achieved even for a misalignment of the package of larger than
50% off the test board pad centres. A small residual displacement
of the package from perfect alignment after reflow is observed.
The reason for this displacement is the action of gas flow viscous
drag on the package during reflow. The use of eutectic SnPb solder
paste slightly reduces self-aligning ability, due to the increase in
the solder volume, which reduces the restoring force. Exposure of
the solder paste to a 25 C and 85% RH humidity environment
also has a detrimental effect on the self-alignment of the BGA
package, due to solvent evaporation and moisture absorption in
the paste causing solderability degradation. The self-alignment of
the package is also affected when there is slow spreading of molten
solder on the pad surface. This is attributed to the reduction of
restoring force due to the decrease in effective wetting surface area
of the board pad
Scanning acoustic microscopy investigation of engineered flip-chip delamination
The rapid uptake of flip-chip technology within
the electronics industry, is placing the reliability of such
assemblies under increasing scrutiny. A key feature of the
assembly process is the application of underfill to reinforce
the attachment of the die to the printed circuit board. This
has been identified in numerous studies as one of the major
ways in which the reliability of the devices can be
improved, by mitigating the coefficient of thermal
expansion mismatch between chip and board. However, in
order for the underfill to be effective in coupling the die to
the circuit board, its adhesion to the passivation layer of the
die and the solder mask layer on the PCB must be
maximised. There is a growing body of literature that
indicates that poor adhesion at either interface
(delamination) as a result of contamination can result in
premature failure of the assembly through stress fracture of
the solder joints.
In order to investigate further the effect of
delamination on the reliability of flip-chip assemblies,
surface chemistry has been used to control the adhesion of
the underfill to the die passivation. This paper reports how
modification of the die surface by the application of a low
surface energy coating, which prevents the strong adhesion
of the underfill, has enabled the selective delamination of
the device at the chip-to-underfill interface. Using scanning
acoustic microscopy (SAM) the effectiveness of this
treatment in creating controlled delamination before and
after thermal cycling has been monitored. The ability to
engineer delamination, can enable experimental studies of
the mechanics of flip chip assembly failure, which
complement current finite element modelling work