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Understanding hidden reactions and the importance of profile in reflow soldering, part 2
Understanding hidden reactions
and the importance of profile in
reflow soldering, part 2
Dr. S. Manian Ramkumar, Mr. Anand Kannabiran and Ms. Aarthi Baskaran, CEMA at RIT, Rochester NY,
USA; Dr. Viswam Puligandla; Mr. Bjorn Dahle, KIC, San Diego, CA, USA
Reflow profile, component mixing of the joint constituents to ensure
In part one of this three-part types and metallization joint reliability. Mixed assemblies can be
series, the important reflow
It is common to encounter a wide variety classified into two categories—forward
of components, both in terms of types compatibility and backward compatibility.
parameters to be considered
and sizes, in a typical electronic product. In forward-compatible ball grid array
when reflow profiling for a This results in differential absorption
assemblies, the paste melting temperature
given product and component
of heat both by the joint and by the is much higher than that of the solder
component body. Heat taken up by the balls. Such assemblies may therefore
mix were identified. In part
joint is the useful heat that contributes suffer from reduced standoff, resulting in
two of the series, other factors,
to the joint formation, while that taken failures during fatigue loads. With Pb-free
such as board metallization,
up by the body is the lost heat. Huge reflow, components are also subjected to
components, especially of ceramic temperatures above the qualified limits
component finish metallization,
substrate, can act as big heat sinks, of Moisture Sensitivity Level (MSL)
mixing of alloys and their effect
resulting in uneven distribution of heat specifications. Most Sn-Pb process
around the component. In large area array compatible components are recommended
on the IMC formation, will
components, it is hard to get uniform to be used with reflow under 245°C.
be highlighted in detail. These heating throughout the array of the joints, This tends to limit the reflow process
constraints have increased the
which may result in differential expansion window for forward-compatible assembly,
of board and the component. introducing a trade-off situation between
importance of the use of reflow
The situation is complicated when the joint’s microstructural uniformity and
profiling to ensure product mixing Sn-Pb and Pb-free parts with
the MSL qualifications. MSL degradation
reliability.
Pb-free and Sn-Pb pastes respectively. by as much as two levels is not uncommon
In mixed-assembly reflow, when and should be carefully evaluated.
the component termination or lead In backward-compatible ball grid array
metallization or solder ball composition assemblies, the reflow temperatures in the
Keywords: Thermal Profiling,
may not be the same as that of the solder Sn-Pb paste manufacturer’s specifications
Reflow Soldering, Reflow
paste, it is important to ensure complete may not be enough to melt the Pb-free
Process Window, Process
Parameters, Intermetallic
Compound
Figure 1. Types of mixed assemblies.
22 – Global SMT & Packaging - August 2008 www.globalsmt.net
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