Understanding the hidden reactions and the importance of profile—part 3
1. Board damage: Very high temperatures structure. Slower cooling rates give rise to Conclusion
and too long dwell times can cause coarse or lamellar structure that are not In view of the complexities of the assembly
solder mask damage or discoloration. very conducive to long-term reliability and process, a comprehensive understanding
Boards can experience internal damage can pose a reliability risk. of the thermal processes is essential. The
in the form of delamination that can An important aspect sometimes following suggestions are aimed at accom-
later result in corrosion and electromi- encountered in the solidification process plishing successful assembly for reliable
gration failures. is ‘under cooling’ or ‘super cooling.’ As electronic products:
2. Component damage: Very high a tem- the liquid melt is being cooled, solidifica-
1. Understanding the paste specifications
perature and long dwell time result in tion does not occur at the freezing point
and component/PCB limitations.
damage to the temperature sensitive but remains in liquid state below that
2. Understanding the phase diagram
components, thinner and low-heat- temperature, and suddenly cools at a
of the metallurgical interconnection
capacity packages reach a temperature temperature well below the freezing point.
system that is involved in the assembly.
higher than what can be tolerated. During this stage intermetallics continue
3. Understanding the preheat, soak, peak
Very low temperature or shorter dwell to grow as rods and platelets, giving rise to
temperature, dwell time and cool down
time results in poor joint formation in an indeterminate joint morphology that
rate and its influence on the final inter-
case of heavier and larger components. could impact reliability under thermal and
connect structure and morphology.
3. Joint structure: Very high temperature mechanical loading conditions. Super cool-
4. Understanding the use of profiling
and long dwell time would result in ing is generally mitigated with addition of
tools and software to achieve optimum
increased solid-liquid reaction. This minor elements to the solder composition.
parameters.
would result in increased dissolution of The mitigation involves optimizing the
pad metallurgy into the solder joint or cooling ramp down to provide a finer mi- In summary, adhering to the recommend-
formation of excessive intermetallics at crostructure without inducing any thermal ed practices, optimizing the conditions
the interfaces leading to brittle joints. shock either to the printed circuit board or for a given product and maintaining and
Increased intermetallic formation also the components. monitoring those conditions are crucial for
results in Pb-rich regions adjacent to success.
the intermetallic layers that may not Rework
be desirable for joint reliability under This is an important area where thermal acknowledgements:
loading conditions. issues dominate. In spite of the best efforts The authors would like to acknowledge the
to produce quality assemblies, occasions
It is important to map the time and
support provided by Mr. Abhinav Ajmera,
arise where a defective component needs
temperature for each product and optimize
graduate research assistant in the Center
to be replaced. The process involves risks
the profile to mitigate the risks. Significant
for Electronics Manufacturing and Assem-
if the parameters are not optimized. Some
differences can exist depending on the
bly, for reviewing the article.
of the prominent risks are secondary
board structure, thickness, thermal planes,
reflow at the neighboring components if
and the component menu.
references
keep-outs are narrow, pad lift off, solder
1. E.J. Cotts, Robert Kinyanjui, Richard
mask discoloration, and damage, etc. Also,
Cool down
Chromik, Anis Zribi, P. Borgesen,
double sided assemblies with back to back
This is also a very critical step in the as-
Chapter 13, Pg 465-494, “Handbook of
components are more prone to damage
sembly process although not always given
lead-free solder technology for micro-
and warrant special attention.
its due consideration. The long-term reli-
electronic assemblies”
The mitigation involves process
ability of the solder joint depends on the
2. C. M. Lawrence, Mingliang L. Huang,
development with precise and accurate
joint microstructure. A fine grain structure
“Microstructural evolution of lead-free
temperature measurement and control for
is generally preferred. Faster cooling rates
Sn-Ag-Bi-Cu SMT joints during aging”,
a given package-board combination. Proper
are recommended for achieving fine grain
IEEE Trans. On Adv. Pkg., Vol 28 No.
shielding of the neighboring components,
1.
controlled hot air flow patterns, and
3. ASM International Electronics Materi-
6a. knowledge of the precise temperatures at
als Handbook
pre-selected and critical ball locations are
important.
Dr. Ramkumar is the director of the Center
for Electronics Manufacturing and Assembly
(CEMA) at the Rochester Institute of
Technology, Rochester NY
6b. 6c.
Mr. Anand Kannabiran was a graduate
research assistant in CEMA and is currently
working as a process engineer in Vansco
Electronics
Ms. Aarthi Baskaran is a graduate
teaching assistant in CEMA
Dr. Viswam Puligandla is
a retired scientist from Nokia
Mr. Bjorn Dahle is president of KIC Thermal
Figure 6. (a) Capacitor crack (b) Solder balling (c) Delamination.
38 – Global SMT & Packaging – September 2008
www.globalsmt.net
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