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How Clean Is Clean Enough?
December 31, 1969 |Estimated reading time: 3 minutes
By Jane Koh
A review of cleanliness standards reveals that there often is no quick and easy answer to cleaning questions.
A common question that rings through our technical support line is, "What is IPC's standard on cleanliness?" This is a simple, straightforward question often asked by industry novices, so a simple, straightforward answer usually is what they want. However, in most cases, this is not specific enough for their individual needs.
To answer this question, simple criteria must be known first: IPC standard being used, residue type, applicable domain and cleanliness criteria. Table 1 answers these questions the old-fashioned way quick and easy.
But do these answers provide the necessary facts? Unfortunately, they seldom satisfy the caller. In fact, these answers generally prompt additional questions, such as: "Is that all?"; "What if the contaminant has more chlorides?"; "What about flux residues in a no-clean process?"; "What if I conformal coat my assemblies?"; or, "What about other non-ionic contaminant?"
Unlike the "good old days" when rosin fluxes dominated the industry, new surface finishes, fluxes, soldering and cleaning systems are flourishing. It has become apparent that there is no "one size fits all" answer. For this reason, standards and specifications emphasize testing protocols to demonstrate reliability rather than a single pass/fail number.
A closer look at the IPC standards specifically IPC-6012, Qualification and Performance Specification for Rigid Printed Boards reveals the cleanliness requirements for bare printed circuit boards (PCB) after application of solder resist, solder or alternative surface coating should be specified in the procurement documentation. This means assemblers have to tell board fabricators how clean they want the bare boards. It also leaves room for assemblers with no-clean processes to specify a more stringent cleanliness requirement for incoming boards.
Assemblers not only need to specify incoming board cleanliness, but also agree with users on the assembled product's cleanliness. According to J-STD-001, unless specified by the user, the manufacturer shall specify a cleaning option (either no-clean or one or two assembly sides to be cleaned) and test for cleanliness (either no testing required, surface insulation resistance test, or test for ionic, rosin or other organic surface contaminants). The cleaning system then is chosen based on the compatibility of the soldering process and the product. Cleanliness testing will depend on the flux and cleaning chemistry used. If rosin flux is used, J-STD-001 provides numeric criteria for Class 1, 2 and 3 products. Otherwise, ionic contamination testing is the simplest and least costly. J-STD-001 also has a generic numeric requirement, as shown in Table 1.
If chloride content is a concern, results from industry studies involving ion chromatography analysis have shown the following guidelines to be reasonable breakpoints for chloride content. Chloride content increases risks of electrolytic failure when exceeding the following levels:
- Less than 0.39 mg/cm2 for low-solids fluxes
- Less than 0.70 mg/cm2 for high-solids rosin fluxes
- Less than 0.75 to 0.78 mg/cm2 for water-soluble fluxes
- Less than 0.31 mg/cm2 for bare boards with tin/lead metallization.
Cleaning discussions often lead to the ultimate answer: True cleanliness depends on the product and the intended end-use environment. But how can it be determined what is clean enough for a particular end-use environment? Through thorough and strict analysis, each potential contaminant and end-use scenario is studied, and long-term reliability tests are run.
But is there an easier way out? The learning curve can be shortened by tapping into others' experience. Organizations such as IPC, EMPF and the Naval Avionics Center have conducted round-robin tests and industry studies on various cleanliness aspects; some of the findings are available in the public domain. These technical papers and handbooks guide an individual or company in understanding the subtle, yet critical, elements of process testing and effects. A good example is the extensive Cleaning and Cleanliness Test Program performed in the late 1980s that was sponsored by the IPC, U.S. Environmental Protection Agency (EPA) and U.S. Department of Defense (DOD). This program investigated new materials and processes that reduce the chlorofluorocarbon (CFC) levels used in electronics manufacturing cleaning processes.
The next big wave in the electronics industry the lead-free solder and halogen-free laminate movements probably will trigger another extensive industry-wide study on cleaning and cleanliness. Until then, readers and callers will need to discern from various sales materials, case studies, reports and guidelines what their individual cleanliness requirements should be, on top of what IPC specifications call for.
JANE KOH is technical project manager for IPC Association Connecting Electronics Industries, 2215 Sanders Road, Northbrook, IL 60062; (847) 790-5349; Fax: (847) 509-9798; E-mail: janekoh@ipc.org.