1.7 Characterization of the SMT Assembly, Test and System Integration Process
Certain conditions of high strain and high-strain rate are known to cause ENIG solder joints to
fail. Therefore, you must use care to avoid excessive shock and bending of the PC board during
assembly, handling, and testing of FCBGAs with ENIG plating. Examples of severe mechanical
loading that produces high-strain and strain rates during PCB assembly are: In-Circuit Test
(ICT), manual connector insertion, PCB edge-guide snap-off, two-sided assembly, and
mechanical assembly. TI recommends appropriate strain and strain-rate characterization on the
PCB assembly process be performed prior to beginning assembly of a new PCB design (see
Figure 2). Additional care should be taken to avoid steps where severe mechanical loads could
potentially impact the reliability of the ENIG solder joint.
PWB flexural loading depends upon system configuration, assembly location, etc. Consequently,
some PCB assemblers characterize PCB strain for all operations associated with mechanical
loading.
Recommendation #1: Prevent bending, flexing and impact loads.
Check the following for potential problem areas and reduce flexion and impact:
• Second side assembly in a two-sided board
• All manual handling processes:
− All rework and retouch processes
− Connector installation
• Board test processes:
− In-Circuit Test (ICT)
− Board Functional Test (BFT), or equivalent functional test
• Mechanical assembly
− Heatsink assembly
• System board integration
• PCI card installation
• DIMM installation
• Snap off of edge guides
NOTE: Assembly processes for different boards and manufacturers vary. Tests such as ICT and
BFT are referred to generically in this document; nomenclature can vary at different
manufacturing sites. In such cases, apply the same requirements to the equivalent test
processes. However, the goal is to characterize all assembly steps involving mechanical loading.
Do not constrain testing to the steps listed above or to perceived high risk areas. The data from
these tests serve as a baseline for future reference.
Recommendation #2: Using special instrumentation, characterize PCB assembly process
to identify high−stress operations.
Some assembly steps that require characterization include the following:
• Surface mount technology (SMT) assembly processes
• All manual handling processes:
− All rework and retouch processes
− Connector installation
• Board test processes:
− In−Circuit Test (ICT)
− Board Functional Test (BFT), or equivalent functional test
• Mechanical assembly:
− Heatsink assembly
− Manual component addition
− Manual component soldering
− Wiring harness attach/insertion
− Packing
• System board integration
• PCI card installation
• DIMM installation
Additional information on predicting and preventing brittle solder joint failures through use of
strain gage testing can be found in Julie Goldstein’s article, included in section 2, References.
Recommendation #3: Application of package−level underfill.
Upon completion of recommendations #1 and #2, additional precautions may be required to
further reduce the chance of solder joint damage brought about by normal assembly and test
operations. In this instance, package-level underfill can be applied to the as-mounted
component to distribute stresses and reduce the effect of forces concentrated on the BGA solder
joint.
Package-level underfill provides mechanical strength and stability for the package and PCB in
the immediate region of the package. The underfill ensures that the PCB and package remains
firmly affixed and protects the solder joints from high-impact forces which can otherwise damage
the solder joint.
1.9 Conclusions
Electroless nickel/immersion gold (ENIG) plated packages are frequently used in
high-performance flip chip packages. ENIG forms brittle intermetallics with solder. Care must
be taken to prevent excessive strain and strain rate (bending and impact) during handling,
assembly and test of ENIG packages. Strain and strain rate can be measured using special
instrumentation. However, even if special instrumentation is not available, inspect and adjust
each operation to avoid potential problems. Lastly, application of package-level underfill
provides additional strength and protection to the solder joints to prevent damage.
TI continues to work with our suppliers and industry experts to identify positive process
improvements with the electroless nickel/immersion gold plating process. This continuous
improvement activity includes identification of superior process chemistries, optimized process
parameters, and more sensitive quality control procedures.
New alternative BGA pad finishes are emerging. TI is currently involved in development and
qualification of alternative finishes to ENIG, which will enable customers to realize greater
robustness and reliability of the FCBGA package. Please contact your TI Field Service Engineer
or Customer Quality Engineer for timing and availability of packages with non−ENIG pad
finishes. |
