-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
STEP 8: Cleaning
December 31, 1969 |Estimated reading time: 5 minutes
As the demand for cleaning increases, some essential rules remain the same. For example, do not combine too many different cleaning processes into one machine using the same cleaning product.
As the North American electronics market shifts to high-reliability manufacturing, the demand for cleaning processes is increasing. One reason is that traditional production-manufacturing lines relied on no-clean assemblies, which now are becoming less reliable. Flux residues are known to change the capacitance of through-connection contact areas, affecting the signal integrity of high-density integrated and high-frequency circuits. Furthermore, inert no-clean resin layers, when exposed to climatic conditions, form dendrites.
As cleaning demands increase, essential rules remain. For example, a board assembly manufacturer should not combine several cleaning processes into one machine using only one cleaning product. Unfortunately, many manufacturers are tempted to do so, as they perceive this as a cost-effective solution.
A typical contract manufacturer (CM) can be confronted with numerous “non-mixable” cleaning processes on the manufacturing floor. Two of the most prevalent examples are mixing solder pallets with PCB defluxing applications, as well as stencil and PCB cleaning processes. Trying to mix one of these processes can lead to cleaning shortcomings and significant quality drawbacks, ranging from cleaning ability and effectiveness to concerns related to material compatibility (Table 1).
Pallet Cleaning and PCB Defluxing
Solder pallets contain flux residues, as do soldered PCBs. Nevertheless, these flux residues not only have a different chemical make-up, but their respective quantities vary greatly. The usual amount of flux that is deposited on a wave solder pallet is 1000× larger than the amount of flux residues removed from an SMT assembly after reflow. This discrepancy is process-related and can not be avoided. As a result, the sheer quantity of flux will affect the ability of any cleaning agent to perform in the long-run, assuming that both pallets and reflowed PCBs are being cleaned in the same process. For those using surfactant-based cleaning technologies, this soon will become evident.
Figure 1a Laminated (l)
Separating a cleaning agent from flux residues can only work well if appropriate cleaning agents are used. It is recommended that potential users install a separate, low-cost cleaning process for pallets, while investing in another for their respective PCB defluxing application. The cleaning agent technology used in each process also should be different. Using a high-end cleaning agent to remove solder pallets would exhaust its ability to provide a typical bath life of 8-12 months. Using a traditional surfactant-based cleaning agent designed for pallet-cleaning applications is more appropriate. Invest resources where cleanliness matters most. Cleanliness requirements for solder pallets are not as stringent, nor do they need to be.
Stencil Cleaning and PCB Defluxing
Newly developed stencil cleaning product technologies that remove raw, unsoldered paste residues have had a low alkaline to neutral pH, allowing for optimal material compatibility. PCB defluxing, on the other hand, consists of removing predominant organic acids. Such chemical constituents are removed using alkaline cleaning agents. Combining both processes must lead to cleaning concerns. It is helpful to predefine cleaning process windows - the wider they are, the better the cleaning process will be.
Figure 1b. delaminated stencil (r).
The amount of raw solder paste removed during a stencil cleaning operation cannot compare to the amount of flux removed from PCBs. Subsequently, excess raw solder paste suspended in the cleaning agent during normal operations could affect PCBs adversely once introduced to a stencil cleaning process. Minute spheres of solder paste also can get trapped under components and lead to functionality failures in the field.
The process window for a defluxing operation consists, in most cases, of elevated temperatures. Applying these process settings to a stencil cleaning process will again provide insufficient cleaning results and other peripheral problems related to high temperatures.
The objective of a recent global lead-free evaluation was to specify a de-fluxing and stencil cleaning process. The process engineer in charge had been mistakenly informed that a single product could serve both of his requirements. After initial stencil cleaning trials and sufficient exposure of the traditional PCB cleaning agent on the stencil mesh, adhesive, and foil, the stencil frame darkened and the foil delaminated (Figure 1a/b). This was traced back to the fact that an alkaline material, typically used for defluxing PCBs, was used for both stencil and defluxing purposes. The customer acknowledged that he must install separate processes (stencil and reflowed misprinted PCB cleaning), supported by products specifically designed for each single task, respectively. Test results for the designated stencil cleaning agent were successfully performed according to customer specifications (Table 2).
Figure 2. SIR test data (35/85 -24 hr.-100 V DC).
Independent surface insulation resistance (SIR) studies on designated coupons further confirmed that the product met standards, and was approved for global use. Stencil construction materials are prone to failure under certain conditions, and can lead to problems with adhesion from foil-to-mesh, and from mesh-to-frame. Increased temperatures typically required for surfactant-based defluxing products accelerate this behavior. Using stencil cleaning products to remove flux residues will provide unsatisfactory results, meaning the process window is too narrow. Consequently, the operator can experience white residues, indicating insufficient cleaning.
Double-sided SMT boards that are misprinted on one side and reflowed on the other require a customized process. Post-reflow defluxing and solder paste removal can be accomplished given the adequate equipment and process window. The user also has to keep the surface finish in mind. Organic solder protection (OSP) layers, for example, can become affected under certain conditions. Temperature, cleaning-agent formulations, exposure times, and other variables can diminish OSPs. Analytical tests also are available to measure the OSP after cleaning.
Conclusion
For many users, the complexity of each individual process can become overwhelming. Proper process support should include an explanation of process limitations as well as assistance in topics related to other process specifics and economic requirements.
Joachim Becht, Ph.D., head of R&D, ZESTRON, may be contacted at j.becht@zestron.com. Harald Wack, Ph.D.,CEO/VP, ZESTRON America, may be contacted at h.wack@zestronusa.com. Umut Tosun, M.S. Chem. Eng., ZESTRON America, may be contacted at u.tosun@zestronusa.com.