Understanding hidden reactions and the importance of profile in reflow soldering, part 2
solder balls. Hence, more constraints specification, the board and component temperatures. The reactions are also
are added to the generic reflow profile MSL, and component qualification influenced by the presence of other
obtained from the paste manufacturer’s temperatures. A detailed discussion on the elements.
specifications. metallurgical transformations that take Tin (Sn) is one of the metals capable
place during reflow soldering is provided in of interacting with almost all the other
Reflow profile and board the next section. metals to form intermetallic compounds.
metallization Hence most solder alloys contain Sn as
Similar to the component-related MSL Interactions of temperature & joint the primary constituent and other metal
issues, PCB laminates also suffer from metallurgy elements as secondary constituents, to
qualification problems with Pb-free Package-to-board interconnection in aid with the mechanical behavior of the
temperatures. Apart from MSL issues, electronic assemblies is generally effected joint. Copper (Cu) and nickel (Ni) form
other failures related to elevated Pb-Free with the type of alloy used for assembly, the second most common elements in the
temperatures for prolonged durations such as eutectic tin-lead, tin-silver-copper, intermetallic composition. In eutectic tin-
include degradation of the PCB material etc. The soldering process involves the lead solder assemblies, lead (Pb) constitutes
and excessive consumption of copper (Cu) formation of a metallurgical bond by one a significant part of the solder joint
to form IMC. Surface finishes have been of the constituents (tin) of the solder alloy matrix but not the intermetallic. Other
developed to minimize the effect of Cu with the pad and component metallurgies. trace elements commonly present in the
consumption by introducing metallurgy The metallurgical bond involves the joint include gold (Au), silver (Ag) and
such as Nickel (Ni) that limits IMC formation of an intermetallic phase at bismuth (Bi). Many of the stoichiometric
formation. Hence, when using a surface the interface of the two metallurgies compounds formed by these elements stay
finish that does not have the mechanism being bonded. Intermetallic compounds dispersed in the joint matrix rather than
to inhibit or minimize excessive IMC are also referred to as stoichiometric the intermetallic. Their presence often
formation with Cu pad, it is advisable to line compounds in metallurgical terms. affects the mechanical property, behavior
use minimal TAL. Intermetallics are in general more brittle and reliability of the solder joint.
than the constituent metals. Therefore, The goal in this section is to examine,
Importance of reflow profiling tools the integrity and reliability of the using phase equilibria diagrams, the
While developing a product-specific reflow interconnection is very much dependent effect of the presence of elements and
profile, it is difficult within the scope, on the thickness and voids, if any, in the their tendency to mix with other metals.
experience and skill to manually generate intermetallic layer. The changes that occur in commonly
a profile. It requires software control that The Stoichiometric line compound encountered solder systems when subjected
is capable of prediction, capability analysis formations are generally initiated from the to reflow and subsequent field usage
and handling large data to correlate and higher temperatures of reflow soldering, are discussed with an emphasis on the
obtain optimum oven settings. Reflow and their growth continues through solid- influence of other elements (from PCB
profiling kits allow numerous thermocouple state reactions at elevated application substrate finish or component termination/
attachments at various locations to
determine the thermal gradient across the
product. Sequential algorithms used in the
tools help to determine the optimal oven
settings during the profiling process. It is
important for the profiling tool to assess
the oven capabilities and limitations.
Reflow profiling includes careful assessment
of several factors, such as the heat capacity
of the components, component density,
PCB size, PCB structure, PCB density and
single- or double-sided assembly. Such
assessment and optimization ensures the
product’s long-term reliability.
Another important aspect in reflow
profiling is the heat capacity of the
packages. If the leads of a package get
hotter sooner than the package or the
board, the solder will wick up, leaving less-
than-optimum solder for joints. This results
in insufficient heel, toe or side fillets. It
is a challenging task to ensure that all
components, big and small, ceramic and
plastic, etc., have the optimum solder joint
shape, morphology and structure.
An ideal process for every product
being manufactured should be developed
by taking into account all limitations
presented in the paste manufacturer’s
Figure 2. Sn-Pb binary system phase diagram.
www.globalsmt.net Global SMT & Packaging - August 2008 – 23
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