Reading time ( words)
With modern environmental restrictions, the electronics industry has moved away from broadly applicable solvents to a host of chemistries than accomplish cleaning within certain parameters. Cleaning materials developers must study the soils found on common PCB assemblies and produce cleaners that will attack residues effectively without damaging component bodies, PCBs, labels, or solder joints. In his recent Boston presentation, Mike Bixenman, D.B.A.., CTO, Kyzen, discussed the new reasons and new ways to clean PCBs.
Lead-free assembly pushed cleaning out of the high-reliability niche and back into mainstream SMT assembly. No longer just for military and medical circuits, cleaning is performed by the 3C sectors: computing, consumer, and commercial. Furthering this trend, miniaturization is increasing cleaning requirements as leads are spaced more closely together. Highly dense advanced packages reduce spacing between I/Os and stand-off heights. Residues can bridge one pad to the next more easily than in previous years. The complexity of removing flux residue increases, while elevating the risk of white residue under low stand-off components. Site-specific failures, caused by a residue left on one pad, or cleaning solution left behind under a component, are on the rise.
To address this concern, matching the cleaning agent to the soil opens the process window. This is not the only way to better cleaning results — designing for cleaning and tailoring the cleaning energy, temperature, and flow also net results — but the materials compatibility and effectiveness are Bixenman’s main focus. He points out that the better the cleaning agent removes residues and reaches all areas of the board, the less time and energy is exerted on the cleaning step. A poor cleaning agent with good cleaning equipment will still take more time and thus use more energy to achieve cleanliness.
The new fluxes for lead-free assembly are designed for miniaturized and lead-free assembly. They leave a greater amount of residue, have reduced volatiles, use weaker organics (in halogen-free fluxes), and offer specific properties like no slump or slow wetting. Each element of these new fluxes requires something of the cleaner. Bixenman estimates that there are about 250 solder pastes on the market.
How does one cleaning material set accommodate all of these demands?
Higher concentrations of cleaning agents in the wash is a logical answer. However, studies show that higher concentrations have no real effect on some lead-free residues; indeed, they can sometimes perform worse than standard solutions.
More aggressive cleaners can attack residues, but they also have the potential to ruin parts labels, marks, and anodized surfaces. Some can even attack the solder alloys they’re supposed to be cleaning.
Wash temperature can change how a cleaner acts; however, this brings us back to changing the equipment. While optimizing directional force, flow and pressure (measure the jet pressure at the PCB surface, not at the nozzle, for more accuracy) are good for the cleaning step, the chemistry must be optimized first.
Trial & error vs. R&D
Typical feasibility tests are performed with only a small sample of an assembler’s product mix. This can lead to surprises in production. Instead, materials analysis can be performed to match cleaning agent to the hardware. Once the researchers believe materials compatibility is achieved, they can run these test vehicles. Don’t start with test vehicles, Bixenman says, use them to verify preceding engineering work.
What else can be done to make more reliable assemblies?
Cleaning also cannot resolve all issues. If cleaning a no-clean assembly is freeing up metallic salts, but not removing them, the cleaning step has actually lowered reliability. Ultrasonic cleaning can be more aggressive than water jets, but the ultrasonic energy may actually damage wire bonds inside packages on the PCB. Solder paste developers are not always designing their alloys for cleanability, with so many other concerns to tackle. In the same vein, reflow/wave under nitrogen can improve the cleanability of an assembly, but may not be specified by the assembler.
At the cleaning step, assemblers should use a materials set that achieves the best results for their solder alloy/flux type, other materials on the board, and cleaning equipment. Concentration, temperature, and flow should be tailored to get the best throughput and cleanliness without damaging the board. Perform appropriate maintenance to keep the cleaning process in-spec.
Meredith Courtemanche, executive editor
For more information about cleaning agents or the cleaning process, contact Bixenman at www.kyzen.com. Read other articles Kyzen authored: Sustaining a Robust Fine-feature Print Process and Cleaning for Reliability Post QFN Rework
Dr. Bixenman will present a paper titled “Improving the R.O.S.E. Process Control and Quality Assurance Method” at the International Conference on Soldering and Reliability (ICSR), which is co-located with the Lead-Free Academy and SMTA Toronto Expo and Tech Forum. The presentation is scheduled to take place on Tuesday, May 18, 2010 at 2:45 p.m. at the Hilton Toronto Airport Hotel in Toronto, ON.
SMT, April 2010