How clean is “clean?” And how can one tell?

What are the required cleanliness levels of printed circuit boards, and how will contamination ultimately affect the long-term reliability of electronic assemblies? This is one of the hot topics in electronics manufacturing. So, how clean is clean? A critical factor to consider is what level of product reliability is required. Answering this question is complex and requires detailed examination.

Numerous methodologies for assessing PCB cleanliness are common practice within the industry. They range from rapid tests that detect only certain types of contamination to more complex methods that are subject to the latest IPC testing standards. All are useful tools for qualitative and quantitative assessments. Which to use is a matter of preference and, of course, the process quality standards and product reliability requirements set forth by the end-customer or OEM.

Visual inspections are performed in accordance with IPC-A-610E, Acceptability of Electronic Assemblies, to detect the presence of visible surface and under-component residues. The results qualitatively assess the presence of residues. Other commercially available and easy-to-use methods include flux, resin, copper, phosphor, halide and ink tests. The ink test is applied to the solder mask and visually qualifies surface cleanliness through the determination of surface energy, while the other tests provide evidence of specific types of residues via color reaction, either on the substrate itself or by using a test strip.

More complex methods include ionic contamination, ion chromatography and surface insulation resistance (SIR), to mention just a few. These tests vary in complexity and depth of analysis, and each is performed in accordance with IPC test methods.

Briefly, ionic contamination testing is a measure of average ionic contamination on the board surface and is generally used to determine if PCBs conform to the requirements of a process performance specification. Ion chromatography measures the levels of anionic and cationic contamination present on a board surface. It is an interesting analytical method that determines the pass/fail limits of each ionic species based not on IPC standards but rather as established on a case-by-case basis. In the absence of industry standards for specific ion levels, the end-customer often defines the allowable limits based on prior experience and the product’s end-use environment. SIR testing helps show the impact of flux residues on the electrical reliability of a device and is often conducted using industry standards. This test is designed to expose a processed or unprocessed printed wiring substrate to elevated temperatures and humidity while applying an electrical potential to determine the propensity for electromigration.

In practice, one or more of these tests are used by manufacturers as a part of their internal quality procedures or as part of a qualification process. Field experience has shown that often manufacturers find existing contract requirements have changed or different demands must be met to earn new business. A thorough understanding of one’s manufacturing process and available test analytics may not be enough to meet the required process demands or test requirements for a new product specification. In these cases, consulting with process experts can be beneficial.

In a case we were involved with, a full-service EMS provider was using ionic contamination testing as part of its quality process for numerous products and consistently exceeding the cleanliness specification of less than 10µg/in2. To secure additional business with a new customer the firm was required to pass an SIR test. Following current practices regarding its cleaning processes, it found that it was unable to pass. The failure was traced to residual flux, particularly underneath components. Thus, even though its core products were manufactured to specification using ionic contamination as a measurement, the qualified cleaning process failed to clean the assemblies to pass the SIR test. Once the failure’s origin had been identified, we helped assess the cleaning process and optimize all parameters so that the EMS firm was able to pass the SIR test and secure the new business. Following the final qualification, the customer incorporated the cleaning process improvements for all products, thereby enhancing the overall product quality for all its customers.

In various other cases, a variety of test techniques were used to evaluate a current cleaning process and assist processors in meeting their cleanliness requirements. Utilizing the types of rapid test techniques mentioned earlier can assist in identifying the presence of residues and the potential for failure and steer one toward an appropriate, more sophisticated test technique.

Other analytical test methods include electrochemical migration resistance and surface organic contamination testing, each performed in accordance with IPC standards. Electrochemical migration testing provides a means to determine the propensity for surface electrochemical migration. It can be used to assess soldering materials and/or processes. Surface organic contamination testing, also referred to as nonionic analysis, is used to determine if organic, nonionic contaminants are present on a bare printed wiring board and completed assembly surfaces.

How clean is clean, and how can you tell? It just depends. Having an understanding of the test techniques available, and the purpose of each, as well as the product reliability requirements, will point you in the right direction.

Richard Burke is national sales manager at Zestron USA (zestronusa.com); richard.burke@zestronusa.com.

When higher preheat temps and longer contact time don’t improve hole fill, what’s next?

Problem

• Wave soldering process cannot achieve topside fillets on thermally challenging assembly.
• Operators manually touch up 100% of solder joints on specific components.

PCB Description

• 3"x12" power management PCB is 150 mils thick with heavy copper planes throughout and ENIG final finish.
• It is densely populated with SMT components on both sides, and contains large PTH rectifiers and electrolytic capacitors.

Process and Equipment

• The process uses SAC 305 solder and a popular, no-clean VOC-free flux designed for Pb-free wave soldering. Local environmental regulations mandate use of VOC-free flux formulations.
• A selective solder pallet that holds two PCB assemblies shields the SMT components and adds additional thermal mass during soldering (Figure 1).
• The PCB is preheated to a topside temperature of 100°-108°C and has 8.3 sec. of wave contact at a conveyor speed of 1.25 ft./min. (Figure 2).
• The wave solder machine is an Electrovert Electra outfitted with a spray fluxer, three bottomside forced air preheaters, three topside Calrod preheaters, and nitrogen-inerted chip and smooth waves.

Diagnosis

• Process engineers have tried improving hole fill by re-profiling to increase preheat temperatures and/or contact time, but cannot get better results. In many cases, the results get worse as more heat or wave contact is added. The process shows classic symptoms of flux burnout.

Improvement strategy

• Switch to a flux that has better thermal endurance and methodically step up the heat in the process, observing changes in PCB temperature and solderability.

Results

Run 1: Change flux; maintain same process parameters and evaluate results.

• No change in topside hole fill
• Indicates flux activity or loading is not a factor

Run 2: Begin increasing preheat temperatures with small step of 30°F per preheat zone.

• Marginal change in topside temperature
• No change in topside hole fill, indicating need for more heat

Run 3: Increase preheat setting by additional 50°F per zone.

• Topside PCB temperature up to 110°C
• Some improvement in hole fill, but not quite yet acceptable

Run 4: Increase preheat settings by another 50°F per zone. Increase flux loading by changing valve factor parameter from 50% to 70%.

• Topside PCB temperature up to 120°C
• Topside temp >100°C for 2 min. to raise PCB core temperature
• Considerable improvement in topside fill, most joints are acceptable

New process:

• The new process maintains the same belt speed of 1.25 ft./min., but now achieves a topside temperature of 120°C and 11.5 sec. of total contact time (Figure 4).
• The VOC-free no-clean flux leaves no visible or palpable residue and provides high post-soldering electrical reliability.
• Quality and throughput are improved; costs and bottlenecks associated with manual touchup are reduced.

Karl Seelig is vice president of technology at AIM Solder; kseelig@aimsolder.com. Carlos Tafoya is technical applications manager at AIM.

What is Flux Burnout?

Flux burnout occurs in wave soldering when the flux’s activators get spent in the preheat portion of the process, before the circuit board reaches the solder wave. It can happen to no-clean and water-washable fluxes, and can present big problems on thermally massive PCB assemblies.

When heated, flux activators begin removing existing oxides from solderable surfaces, and continue removing new ones that form during the heating process. They should remain active throughout the soldering cycle to facilitate wetting, but have a finite lifespan. If the activators are fully expended during the preheat cycle, new oxides build up and hinder joint formation.

PCB assemblies with high thermal mass challenges – design elements like thick copper planes, bulky components or poor thermal relief on ground ties – need extended preheat cycles to warm them to soldering temperature and extended wave contact times to let solder wick up the holes. It is not uncommon for thermally challenging assemblies to experience flux burnout, especially with the slower conveyor speeds of Pb-free wave soldering processes.

Diagnosing flux burnout. Try this simple test: slow the wave solder machine’s conveyor speed and examine hole fill.

• If slowing the conveyor improved hole fill, the flux was still active. The increased preheat and contact time resulted in better hole fill.
• If slowing the conveyor did not improve hole fill, the flux was spent. The flux stopped cleaning the oxides before soldering was completed.

Most wave soldering fluxes are designed to maintain activity and reliability across a wide process window, from fast and cool profiles to slow and hot ones. When high thermal mass PCBs demand extreme time-temperature exposure, typical flux activators may not survive. Specialized activators with enhanced thermal endurance are needed to ensure good solder wetting, acceptable hole fill and reliable mechanical performance.

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