Wednesday, June 20, 2018
Publication Date: 07/1/2011
Archive >  July 2011 Issue >  Special Features: SMT and Production > 

Keep Your Cleaning Processes Separated
Delaminated stencil foil.

The shift toward high-reliability applications has put increasing demands on cleaning processes to ensure that the high-reliability remains after the product leaves the assembly facility. One of the main factors is that up until now, traditional 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 layers — when exposed to climatic conditions such as temperature fluctuations 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 manufacturer can be confronted with numerous "non-mixable" cleaning processes on the manufacturing floor. Two of the most prevalent examples are combining 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 problems and tremendous quality issues. These can range from cleaning ability and effectiveness to material compatibility concerns.

Solder Pallet Cleaning
Solder pallets contain flux residues as do soldered printed circuit boards. However, solder pallet flux residues have a different chemical make-up and more importantly, the residue quantities are much larger as well. This discrepancy is simply process related and cannot be avoided. As a result, the sheer quantity of flux will significantly affect the ability of any cleaning agent to perform in the long-run, assuming that both pallets and reflowed printed circuit boards are being cleaned in the same process.

This will very quickly become evident for manufacturers currently using surfactant-based cleaning technologies. The separation of cleaning agent from flux residues can only work well if the appropriate cleaning agents are used. It is highly recommended 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 surfactant-based cleaning agent that has been designed for pallet cleaning applications is much more appropriate and certainly more cost-effective.

Therefore, investing your limited 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.
Stencil compatibility and cleaning results with VIGON® SC 202 at 25 percent concentration.

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 defluxing, on the other hand, predominantly consists of the removal of organic acids. As such chemical constituents are only effectively removed with alkaline cleaning agents, combining both of these processes will ultimately lead to cleaning problems. Predefining the cleaning process window can be very helpful. Generally speaking, a wider process window leads to more flexibility within the cleaning process and ultimately, better results. The amount of raw solder paste removed during a stencil cleaning operation is significantly higher than the amount of flux removed from PCBs. The raw solder paste excess, which is suspended in the cleaning agent during normal operations, could adversely affect PCBs once introduced into a stencil cleaning process for defluxing. To make matters worse, minute spheres of solder paste (solder balls) can be easily trapped under components and lead to product failures in the field.

Elevated Temperatures
Furthermore, defluxing operations are typically conducted at elevated temperatures. Applying these process settings to a stencil cleaning process can lead to stencil material compatibility problems related to high temperatures.

During a recent global lead-free evaluation, the objective was to specify a defluxing and stencil cleaning process. The process engineer in charge had been mistakenly informed by other industry sources that a single product could meet both requirements. After initial stencil cleaning trials and sufficient exposure of the traditional PCB cleaning agent to the stencil mesh, adhesive and foil, the stencil frame darkened and the foil delaminated. This was traced back to the fact that an alkaline material, typically used for defluxing of printed circuit boards, was suggested to the customer for both, stencil cleaning and defluxing purposes. The customer acknowledged that the only solution would be to install two separate processes (i.e. stencil and reflowed misprinted PCB cleaning) and use cleaning agents specifically designed for each task. The designated stencil cleaning agent was tested according to the customer's specifications.

Furthermore, independent SIR (surface insulation resistance) testing with designated SIR coupons validated the product's cleaning performance in accordance with Belcore Standard GR-78 CORE, Issue 1, Section 13.1.3 and was subsequently approved for global use.

Stencil Cleaning Care
Stencil construction materials are typically prone to failure under certain conditions and can lead to problems with adhesion from foil to mesh and from mesh to frame. Increasing the temperature, which is typically required for surfactant-based defluxing products, accelerates this delamination process. On the other hand, using stencil cleaning products to remove flux residues will provide unsatisfactory results, i.e. the process window is too narrow. Consequently, white residues may be left on the substrate's surface, which is a clear indication of insufficient cleaning.

There is also a generally overlooked cleaning process. For double-sided misprinted SMT assemblies, which are misprinted on one side and reflowed on the other, oftentimes a customized process is needed. Here, post-reflow defluxing and solder paste removal can actually be accomplished, given adequate equipment and process window. However, the user has to consider the surface finish at all times. Organic solder protection (OSP) layers, for example, can become affected under certain conditions. Temperatures, cleaning agent formulations, exposure times and other variables can easily diminish OSPs.

There are specific analytical tests, such as OSP thickness measurement equipment, to measure the organic solder protection layer after the cleaning cycle in order to ensure that the assemblies are still viable.

Dealing with the complexities of cleaning processes as well as the task of setting up the most economical production line can become a monumental task for some manufacturers. Industry cleaning experts have set up state-of-the-art technical centers where proper process support includes hands-on cleaning demonstrations and trials. This help should include the assessment of any process limitations as well as ample assistance in topics related to other process specifics as well as economic requirements. Such a methodical approach can help engineers and companies differentiate between short-term as well as long-term cost savings and ultimately lead to a successful cleaning process.

Contact: Zestron America, 11285 Assett Loop, Manassas, VA 20109 703-393-9880 fax: 703-393-8618 E-mail: Web:

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