|Before and after views of the result of effective cleaning.
Component manufacturers are continually developing new and smaller packages for components that are mere fractions of a millimeter and have board-to-component clearances of less than 1 mil. Pick-and-place machines offer new accessories that allow placement of these almost invisible parts. Components are placed so close together that effectively cleaning under these parts would appear difficult.
The use of lead-free solder in electronic circuits and assemblies has necessitated new solder, new fluxes, higher processing temperatures, and new solder processing equipment. Many new approaches, alloys, chemicals, and soldering processes have been developed to address these issues. Tin whisker problems have also increased dramatically. Time-delay effects, however, often will not surface until an item is being produced and has been in service for even a year or more. Product development rates, with new models available every year, can cover up these time-delay issues. For some electronic products, such as cellular telephones, problems may not surface since few people are using a two-to-three-year-old mobile phone. Repair of an old mobile phones may not be considered a viable option when an upgraded feature-rich model is available at a subsidized price. For medical devices, however, the potential for failure is very real and the effects of failure can be devastating.
Time-delay problems can be addressed on the manufacturing floor, but often component manufacturers, board designers, and manufacturers operate independently of one another and there is a lack of communication about any potential problems. Research shows that many failures are a result of printed circuit boards (PCBs) that have not been adequately cleaned of contaminants left over from the manufacturing process. There may also be some difficult design issues for a PCB whose close quarters have resulted from the use of the advanced computer-aided-design (CAD) software used to design the PCB, such as ground points too close to a power bus. Many designers have little exposure to the production issues of PCB fabrication or board assembly. It's important for a board designer to understand how flux from the hand soldering process of a connector can flow to a microviahole placed near a connector pad. The design of a PCB can have considerable impact on the other processing steps used to manufacture that PCB.
IPC (www.ipc.org), the Association Connecting Electronics Industries, is a global trade association serving the PCB and electronics assembly industries, their customers, and suppliers. It has a task group devoted to addressing all topics connected with determining the cleanliness levels of unpopulated (bare) printed circuit boards and has established a base standard for cleanliness. The IPC-TM-650 standard sets an acceptable range of 10-2 µg/in. 2 of sodium chloride (NaCl). Is this standard enough to prevent failures and can today's cleaning methods truly clean the boards that are being produced right now?
There are a number of myths circulating in the industry:
- All bare boards coming from the fabrication house are clean.
- All components are delivered clean with no contamination issues.
- Flux will never present any problems and can just be poured onto the board without worrying about heating or the absence of heat and everything will work out okay.
While this is not the reality in manufacturing today, the assumption of these concepts presents major issues for manufacturers. A circuit board with hidden residual flux contamination may pass quality control (QC) and operate properly. After arriving in its operating environment, there may be high humidity and temperature swings that generate condensation that initiates residual flux problems that grow and eventually cause leakage paths that ultimately cause failures. The high-impedance circuits of today's micropower electronics are even easier to disrupt with stray voltage sources.
Washing a PCB following the soldering process typically produces a board that appears clean. But contamination may still be present in areas that are not visible. A conventional wash cycle may not be sufficient to clean the board. Rather, it may require a combination of chemicals, temperature, wash cycles, and timing to get some boards really clean.
"The cleanliness of a printed board can directly impact the effectiveness or quality of an assembled printed board," said John Perry, IPC technical project manager and staff liaison to IPC's Bare Board Cleanliness Assessment Task Group. "Residues increase the risk of field failures or can electrically impede a printed board's function, so having acceptance criteria for various levels of testing as well as direction on how many samples should be tested is extremely important."
Digicom Electronics has used significant resources to research these issues. In many instances, it was found that a combination of comparatively minor points, when combined, pointed to processes that simply do not work as well in an up-to-date manufacturing environment, although they provided acceptable results in the past. Minor changes in component packaging design, materials, CAD/CAM software, board fabrication, and chemistry have combined to slowly change the robustness of the manufacturing process. Even the best intentions have often unwittingly created the potential for defects to occur in electronic circuits in ways not previously possible. It was only through a comprehensive evaluation of all materials and steps in the manufacturing process that root causes of potential problems were discovered. Armed with a clear picture of potential problem areas, much effort was expended to find solutions to each problem.
At Digicom, new equipment and supporting subsystems were ordered and installed. Test and evaluation procedures were developed and implemented to verify the effectiveness of each process. All boards were routinely run through the system both prior to and following the assembly process to ensure that the soldering process was not compromised by any contaminants from the PCB fabrication process and that the PCBs were completely clean of any remaining chemicals from soldering.
This cleaning process is completely "green." De-ionized water from polishing tanks is used and recycled. Filters catch the solids while powerful blowers ensure that harsh chemicals don't migrate past the holding tank. Clear windows on the equipment enable the operator to monitor the entire process. A refractometer checks the stability of the mix in the tank to verify that it's not compromised. The discharged liquid is completely environmentally friendly.
Processes were then thoroughly tested, evaluated for effectiveness, and ultimately validated as a stable process. Independent tests showed that circuit boards that went through this cleaning process tested 75 percent cleaner than the 10-2 µg/in. 2 specified by IPC as its highest level of cleanliness. In addition, a lab analysis for ion contamination found zero levels of NaCl ion contamination on the assembled boards.
This level of cleaning eliminates failures caused by board contamination. Products are less susceptible to corrosion-induced failures, thus reducing the need for maintenance or repair. In addition, the cost savings to companies, especially those with military, aerospace and medical device applications, are considerable.
Contact: Digicom Electronics, Inc., 7799 Pardee Lane, Oakland, CA 94621 510-639-7003 E-Mail: email@example.com Web: http://www.digicom.org