Board cleaning is perhaps one of the most overlooked aspects of printed circuit board (PCB) assembly. But savvy medical electronics original equipment manufacturers (OEMs) have a keen sense of the importance it plays to ensure their products are free of miniscule flaws, defects, debris, and dust. In special cases, critical examinations must be performed to assure the correct cleaning solvents and processes are implemented to avoid damaging contaminants.
This new level of medical electronics PCB scrutiny and resolution is best described as a design of experiment (DOE). DOEs extend well beyond the extensive nature of PCB cleaning to also investigate and solve unforeseen PCB design/layout, fabrication, and assembly issues. These unpredictable and quite unexpected problematic areas usually cause the OEM to abandon a costly PCB investment simply because the electronic manufacturing services (EMS) provider cannot locate and define a particular issue.
Advanced medical electronics PCBs are especially subject to these particular sensitive issues. So, OEMs are taking an even closer look at today’s board cleanliness and demanding ultra-clean boards due to newer and smaller packaging and ever-shrinking PCBs. In particular, they want to avoid product flaws or failures occurring due to either new cleanliness challenges or the use of inadequate PCB cleaning solvents and practices.
In some cases, OEMs want to evaluate conventional de-ionized (DI) water with newer, different chemicals for batch cleaning, test the results, and perform Ionograph testing to determine which cleaning methodology is best for their products. (See Figure 1)
Bubbles Creep in
While standard PCB cleaning practices are adequate in most instances, there are times when the EMS provider and medical electronics OEM must take a step back, such as when an unexpected problem occurs due to insufficiently clean sub-assembly circuits. At that point, both the EMS provider and the medical electronics OEM should work closely to begin to formulate the DOE.
For example, an ultra-high frequency flex circuit that showed bubbles created during assembly is shown in Figure 2. Flux creepage or perhaps some water penetrated through the flex circuit membrane or solder mask to create those bubbles, thus making those assemblies useless.
Like this particular sub-assembly, growing numbers of medical electronics PCB designs are high, and ultra-high frequency assemblies that demand special cleaning processes. And like the flex circuit discussed here, some PCBs for handheld and portable applications are so extensively and densely populated with small, complex packages in a remarkably small area that DOEs must be ushered in to resolve design or assembly issues, such as cleaning. First, however, special cleaning processes must be applied before DOE practices are exercised.
Special cleaning processes for high-frequency assemblies, including those for medical electronics PCBs, have the following target requirements.
(a) The removal of ionic contaminations to the highest possible level, basically, to make the assembly in the purest form as much as possible.
(b) Use of a low surface tension solvent to achieve better penetration specifically under the low profile component packages. These miniscule packages don’t have too much room or have too thick or too big of a ball or bump. Low surface tension solvent is necessary so that those chemistries can go underneath those small devices and do the extra required cleaning.
(c) Use environmentally sensitive material, something that is not too corrosive or detrimental to the environment. Some chemicals used in some special cleaning chemistries could include a combination of a harmful solvent such as sulfuric acid, which is detrimental to the environment. Hence, careful choices about certain environmentally sensitive materials must be made.
(d) These solvents should be used with ultrasonic cleaning at stage one or first pass cleaning, and ultrasonic machines should be used.
(e) If the cleaning isn’t performed as per the customer’s requirement, the second stage of ultrasonic cleaning is conducted using de-greaser machines to do the job. (See Figure 3)