Intermetallic structures in solder joints are intrinsic and necessary for the formation of a reliably strong bond between the solder and the connective joint or pad. Without the complex interaction of metals forming the physical structures responsible for the “adhesive” properties of the alloy, there would be a lack of anchoring sites needed for proper bonding. Intermetallic compounds (IMCs) occur from the migration of elements of the alloy with a variety of oxidation states during the various stages of heating and cooling. As a consequence, the same mechanisms responsible for creating a robust connection can also result in the development of joint embrittlement.

The oxidation state of the IMC is as critical to the reliability of the solder joint as the quantity or thickness of the IMC formation. Not all IMCs are created equal as far as constructing a good, strong solder connection. In a simple binary system (such as SnPb), the interaction with copper can form two IMCs: Cu6Sn5, and a secondary phase of Cu3Sn. The growth of the IMC is a natural progression dictated by a number of factors and most commonly follows a parabolic derivation of the growth kinetics equation. One of the primary factors that dictate the extent of intermetallic growth is the initial thickness of the intermetallic at the time of formation. If the thickness of the intermetallic approaches 5 microns, there is a risk of joint failure due to IMC embrittlement. This necessitates following the proper thermal profiles prescribed by the solder manufacturers to prevent excessive formation of IMCs.

In Lead-Free alloys, the creation of IMCs can be more complex in nature. The ternary composition of many of the Lead-Free alloys incur additional combinations of IMCs, which may impact product reliability. There have been, and continue to be, efforts to characterize the physical nature and effect of IMCs of the many Lead-Free solder candidates available in the market. The large number of unknowns associated with the use of multi-component solders increases the difficulty in analyzing the effects of IMC formations. Mixed solders are often encountered in re-work circumstances or in situations where components, such as BGAs, have been incorrectly procured and allowed to slip into the product stream. As a result, the manufacturing process engineered for a specific alloy must now contend with the prospect of an alloy with different thermal profile characteristics. This can encourage the growth of intermetallics due to the improper thermal conditions promoting excessive IMC growth.  Even under circumstances where the individual alloy constituents are known and the correct thermal parameters are utilized, the matrix of IMC formations can be extensive.

Several distinctive boundary layers of intermetallics can form at different locations across the BGA along with intermetallic particles that are characteristic of the morphology associated with Lead-Free alloys. There is a  potential for reliability issues due to the many intermetallics formed. Since intermetallics continue to grow at a rate dictated by the kinetics of parabolic growth, the initial thickness of the boundary layers show potential for excessive IMC growth and subsequent embrittlement.

In our EMPF Lead-Free Manufacturing course, the students are given the training to understand the processing and material issues associated with a successful Lead-Free assembly. The EMPF has been involved with Lead-Free manufacturing and understands the ramifications of Lead-Free materials for military and
commercial systems. As a member of the NCMS and JG-PP consortium groups that were tasked to study the manufacturing and process issues associated with the Lead-Free transition, the EMPF is an important leader in the role of keeping both our DoD and commercial customers informed.

For more information about our Lead-Free classes or other electronics training, please contact the EMPF at (610) 362- 1320 or via email, registrar@empf.org.