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)