Overcoming the Potential Failures of Electronic PCB Assemblies

Overcoming the Potential for Failures

Component manufacturers are constantly developing smaller and innovative packages for components that are just fractions of a millimeter and have board to component clearances of less than a mil. New accessories that allow placement of these nearly invisible parts can be found in pick and place machines. When the components are placed very close together, how does one effectively clean under something that is extremely small?

No lead solder is a relatively recent legislated fact of life that necessitated new fluxes, new solder, new solder processing equipment and higher temperatures. In order to address these issues, several new approaches, chemicals, alloys and soldering processes have been developed. Tin whisker problems also increased considerably.

However, time-delayed effects usually will not occur until a product is out the door and has been in service for even a year or two. Some of these time-delayed issues are covered up through the pace of product development when products are routinely discarded for newest models. Products such as mobile phones do not face these problems often because relatively few people are using a mobile phone that is over 2-3 years old. When an upgraded feature-rich model is available at a subsidized price, few will spend money for repair. However, for medical devices, the possibility for failure is very real and the effects of failure can be devastating.

Design Decisions Impact Other Process Steps

The manufacturing floor is the single place where it all comes together and where the supporting elements have to operate properly. However, device manufacturers, board designers and component manufacturers are operating independently of one another and this communication void worsens the problem. Research shows that many failures are caused by PCBs (Printed Circuit Boards) that are not clean enough of contaminants from the manufacturing process. There are design issues that are enabled by the sophisticated CAD design systems utilized for PCB design.

A designer will sometimes employ features such as ultra-close copper fill or pour that puts around a few mils from a power bus all over the board. The opportunity for shorts with disastrous consequences just increased several times. A lot of designers have little exposure to the production issues of board assembly or PCB fabrication. A board designer should actually understand how flux from the hand soldering process of a connector can flow to the microvia that was located immediately next to a connector pad. The other process steps will be affected by several decisions made while implementing a design.

Standards Need to Be Raised for High Reliability

The global trade association, IPC serves the printed board and electronics assembly industries, their suppliers and customers. It has a task group dedicated to addressing all topics related to establishing the cleanliness levels of unpopulated (bare) printed circuit boards and has determined a base standard for cleanliness. The IPC-TM-650 standard sets a tolerable range of 10-2 micrograms/in2 for military applications (65-2 micrograms/in2 for general applications) of sodium chloride (NaCl).

Is this standard enough to prevent failures and can present cleaning methods truly clean the boards that are being produced today?

There are several myths circulating in the industry:

1. All components are delivered clean without any contamination issues.

2. All bare boards coming from the fab house are clean.

3. Flux will never present any issues and can simply be poured onto the board without worrying about heating or the absence of heat and everything will work out okay.

Although this is not the reality in manufacturing today, the assumption of these concepts are of major concern for the manufacturer. A board with hidden residual flux contamination is likely to pass QC and operate correctly. After arriving in its operating environment, there may be temperature swings and high humidity that produce condensation leading to residual flux problems to literally start to grow and ultimately cause leakage paths that eventually result in failures. Moreover, it is easier to disrupt the high impedance circuits of today’s micropower electronics with stray voltage sources.

Simple Washing Is Not Enough

When a printed circuit board is washed after the soldering process, a board is usually produced that sparkles and looks ready for the next step of its journey to the end user. The areas that are not visible contain the details that spoil this sparkling clean picture. Putting the boards through a wash cycle alone is not enough. A unique combination of chemicals, wash cycles, timing and temperature is required in order to get the boards really clean.

The cleanliness of a printed board can directly impact the effectiveness or quality of an assembled printed board. 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.

John Perry, IPC Technical Project Manager and Staff Liaison to IPC’s Bare Board Cleanliness Assessment Task Group

Significant resources were utilized by Digicom Electronics to research these issues. In many cases, a combination of relatively minor points, when combined, pointed to processes that merely did not work in today's manufacturing environment, although their performance in times past was adequate. The combination of slight changes in CAD/CAM software, materials, component packaging design, board fabrication and chemistry gradually change the robustness of the manufacturing process.

Often, even the best intentions have inadvertently created the potential for defects to show up in ways that were not possible earlier. A detailed evaluation of all steps and materials in the manufacturing process alone helped to discover the root causes of potential problems. Armed with an obvious picture of potential problem areas, considerable effort was spent to come up with solutions to each problem.

Latest equipment and supporting subsystems were ordered and installed. Test and evaluation procedures were exclusively designed and implemented to verify the effectiveness of each process. All boards were routinely run through the system before and after the assembly process to make sure that they were entirely clean of any residual chemicals from soldering and that the soldering process was not affected by any contaminates from the board fabrication process.