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 a mil. Pick and place machines have new accessories that allow placement of these almost invisible parts. Components are placed extremely close together. How do you effectively clean under something so small?
Lead-free solder is a relatively recent legislated fact of life that necessitated new solder, new fluxes, higher temperatures, and new solder processing equipment. Many new approaches, alloys, chemicals, and soldering processes have been developed to address these issues. Tin whisker problems also increased dramatically. Time-delayed effects, however, often will not show up until a product is out the door and has been in service for a year or two. The pace of product development covers up some time-delayed issues when products are routinely discarded for newer models. For products like mobile phones, problems don’t often show up because comparatively few people are using a mobile phone that is more than two to three years old. Why repair when an upgraded feature-rich model is available at a subsidized price? For medical devices, however, the potential for failure is very real and its effects can be devastating.
Stopping a Problem Before It Starts
The place where it all comes together and where the supporting elements have to operate properly is the manufacturing floor, but often component manufacturers, board designers, and the device manufacturer are operating independently of one another. This communication void exacerbates the problem. Research shows that many failures are a result of printed circuit boards (PCBs) not being clean enough of contaminants from the manufacturing process.
There are design issues that are enabled by advanced CAD design systems utilized for PCB design. Sometimes a designer will utilize features such as ultra-close copper pour or fill that puts a ground a few mils from a power bus all over the board. The opportunity for shorts with catastrophic consequences just increased several fold. Many designers have little exposure to the production issues of PCB fabrication or board assembly. It’s important for a board designer to really understand how flux from the hand soldering process of a connector can flow to the microvia placed right next to a connector pad. Many decisions one makes in implementing a design will impact the other process steps.
IPC – Association Connecting Electronics Industries®, Bannockburn, IL, the global trade association, 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 65 down to 2 micrograms/ sq. inch of sodium chloride (NaCl). Is this standard enough to prevent failures? Can present cleaning methods truly clean the boards that are being produced today?
There are a number of myths circulating in the industry:
- All bare boards coming from the “fab” 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.
This is not the reality in manufacturing today, but the assumption of these concepts presents major issues for the manufacturer. A board with hidden residual flux contamination may pass Quality Control and operate properly. After arriving in its operating environment there may be high humidity and temperature swings that generate condensation causing residual flux problems to literally begin to grow, eventually causing leakage paths that can, ultimately, cause failures. The high impedance circuits of today’s micropower electronics are even easier to disrupt with stray voltage sources. (See Figure 1)
Washing a printed circuit board after the soldering process typically produces a board that sparkles and looks ready for its next step. The areas that are not visible contain details that spoil this sparkling clean picture. It’s not enough to put the boards through a wash cycle. It takes a special combination of chemicals, temperature, wash cycles, and timing to get the boards really clean. (See Figure 2)
“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.”
Tackling the Problem
Companies have started to research these issues and some have invested significant resources to find the best results. In many instances, it was found that a combination of comparatively minor points, when combined, pointed to processes that simply did not work as well in today’s manufacturing environment, though their performance in times past was adequate. Minor changes in component packaging design, materials, CAD/CAM software, board fabrication, and chemistry 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 ways not previously possible. Through a comprehensive evaluation of all materials and steps in the manufacturing process, root causes of potential problems can be discovered. Once the problem areas are understood, it is easier to find solutions.
Armed with this information, one electronics manufacturing services company decided to see if it could improve board cleanliness to reduce device failures. 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 after the assembly process to ensure that the soldering process was not compromised by any contaminates from the board fabrication process and that the boards were completely clean of any remaining chemicals from soldering. (See Figures 3a and b)
The cleaning process was designed to be completely green. De-ionized water from polishing tanks was used and recycled. Filters caught the solids while powerful blowers ensured that harsh chemicals didn’t migrate past the holding tank. Clear windows on the equipment enabled operators to monitor the entire process. A refractometer checked the stability of the mix in the tank to see that it was not compromised. The discharged liquid was completely environmentally friendly.
Processes were thoroughly tested, evaluated, and ultimately validated as a stable process. Independent tests showed that boards that went through this cleaning process tested 75 percent cleaner than the acceptable range specified by IPC as its highest level of clean. In addition, a lab analysis for ion contamination found zero levels of sodium chloride 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. When we’re looking at the devices now being made for the medical device industry, including all the wireless and implantable devices, it is time to take a new look at cleanliness standards and the manufacturing processes used to ensure that these products do not fail.
This article was written by Mo Ohady, General Manager, and David Estes, Chief Engineer, Digicom Electronics, Oakland, CA. For more information, Click Here " target="_blank" rel="noopener noreferrer">http://info.hotims.com/49746-164.