DEFLUXING OF EUTECTIC AND LEAD-FREE ASSEMBLIES IN A SINGLE CLEANING PROCESSAuthors: Harald Wack, Ph.D., et al.
Date Published: 6/21/2007 Conference: Symposium
When implementing lead-free solder pastes, users’ assemblies have to be reflowed at higher temperatures, thus intensively baking-in flux residues. To improve the flow of solder at higher reflow temperatures, lead-free pastes contain increased activator content in return impacting the potential risk of corrosion. Furthermore, lead-free pastes also have increased rosin content to achieve low void rates increasing the amount of residues to be removed. Finally, several studies by the authors have determined that silver, a commonly used alloy in lead-free solder pastes, has a tendency to form dendrites.
To prepare for the new cleaning demands, the precision cleaning industry must ask itself what changes to existing processes are necessary to meet the new cleaning demands. In fact, more than 15 years ago, non-ozone depleting legislation forced radical changes—changes that mirror the one presented by WEEE / RoHS now.
Current legislation restricts the level of lead in lead-free assemblies to exceed 0.1% by weight.
A large number of users have previously used DI waterbased cleaning processes, in which water-soluble eutectic products were cleaned. With the onset of lead-free alloys, the authors have experienced an increased demand for cleaning water-soluble lead-free products, as some residues have become water insoluble (due to lead-free soldering process specification). As a result, modifications to the current DI-water processes are necessary, and will involve chemically assisted cleaning. These new users as well as current users are now confronted with more questions, especially whether additional hardware is necessary, or whether processes need to be separated.
Current legislation restricts the level of lead in lead-free assemblies to exceed 0.1% by weight. This limitation was adopted in December 2004, and also included other heavy metals such as Cadmium (0.01%), Mercury (0.1%), hexavalent chromium (0.1%).
Such stringent limitation requires tight process controls, including verification of RoHS and WEEE conformance for the cleaning process. This DOE study was therefore initiated, and begins to shine light on the potential risks associated with cleaning lead-free and eutectic assemblies in a single process also known as mixed cleaning process. An initial customer bath sample analysis was conducted to determine the levels of lead and other heavy metals commonly found in eutectic processes using MPC®-based products. It was found that lead and tin were present in various amounts.
Tin and lead is a commonly known redox-pair with sufficient redox potential to facilitate the reduction of lead on lead-free assemblies. The authors decided to study this phenomenon in more detail and include the ionic contamination pathway as well.
As a result this study was able to address the two most probable pathways to the incorporation of lead into lead-free assemblies.
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