July, 2011
www.us-tech.com
Page 55
Keep Your Cleaning Processes Separated T
By Dr. Harald Wack, President, Zestron
he shift toward high-reliability applications has put increasing demands on cleaning processes to ensure that the high-reliability remains after the product leaves the
avoided. As a result, the sheer quan- tity of flux will significantly affect the ability of any cleaning agent to perform in the long-run, assuming that both pallets and reflowed print- ed circuit boards are being cleaned in the same process.
This will very quickly become evident for manufacturers currently using surfactant-based cleaning tech- nologies. The separation of cleaning agent from flux residues can only work well if the appropriate cleaning agents are used. It is highly recom- mended that potential users install a
separate, low-cost cleaning process for pallets, while investing in a state-of- the-art cleaning process for the PCB defluxing application. Typically, the cleaning agent technology used in each process is very different. Using a high-end cleaning agent for the removal of solder pallet flux residues would simply exhaust cleaner’s ability to provide its typical bath life of 8 to 12 months. Using a traditional surfac- tant-based cleaning agent that has been designed for pallet cleaning applications is much more appropri- ate and certainly more cost-effective.
Therefore, investing your limit- ed resources in the process where cleanliness matters most — the PCB assembly — is well worthwhile. The cleanliness requirements for solder pallets are simply not as stringent as those for PCBs.
State-of-the-art stencil cleaning product technologies to remove raw, unsoldered paste residues have always had a low alkaline to neutral pH, thereby facilitating optimal material compatibility. PCB deflux- ing, on the other hand, predominant-
Continued on next page
Delaminated stencil foil.
assembly facility. One of the main factors is that up until now, tradi- tional production lines have largely relied on no-clean assemblies. However, field experience has shown that long-term reliability of no- cleans cannot be guaranteed. It is well known that flux residues change the capacitance of through-connection contact areas, which primarily affects the signal integrity of high density integrated and high frequency circuits. Furthermore, inert no-clean resin lay- ers — when exposed to climatic condi- tions such as temperature fluctua- tions and humidity — can rupture, which will expose flux activators to the atmosphere and in turn, this can result in the formation of dendrites. As the demand for cleaning increases, some essential rules remain the same. For example, the fact that one should not combine too many different cleaning processes by using one single piece of equipment and only one cleaning product. Unfortunately, many manufacturing companies are still tempted to do so as they perceive this solution as a cost-saving measure.
A typical contract manufactur- er can be confronted with numerous “non-mixable” cleaning processes on the manufacturing floor. Two of the most prevalent examples are com- bining solder pallet cleaning with PCB defluxing applications as well as mixing stencil and PCB cleaning processes.
Trying to mix processes can lead to significant cleanliness prob- lems and tremendous quality issues. These can range from cleaning abili- ty and effectiveness to material com- patibility concerns.
Solder Pallet Cleaning
Solder pallets contain flux residues as do soldered printed cir- cuit boards. However, solder pallet flux residues have a different chemi- cal make-up and more importantly, the residue quantities are much larger as well. This discrepancy is simply process related and cannot be
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