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Engineering Reliability Means Credibility
By Andy Naisbitt, Operations Director, Gen3 Systems G
en3 has been engineering and refining the CM series contaminometer for more than 40 years. Like all measurement systems, the difficulty lies in what is being measured. For example, light is measured by how
many candles produce a certain level of luminosity. But, what if the measure- ment must identify a potential impact on a process or product reliability? In the car industry, metal cannot be painted when the surface dew point
is closer than 5.4°F (3°C) to the ambient air temperature. This measurement is critical to the process finish, as the paint will not adhere well, due to an in- visible layer of moisture on the surface. The measurement is performed with a dew point meter that ensures the metal surfaces are 5.4°F (3°C) above the dew point prior to painting. The GEN3 CM series contaminometer measures the conductivity for an
equivalent weight of salt within a square centimeter of the circuit board to measure the effectiveness of a cleaning process for removing ionic contamina- tion from a PCB.
Why Measure? The closer the pitch of devices in today’s manufacturing — 0.012 in. (0.3
mm) or less — increases the electric field (E = V/d), which makes the electro- chemical corrosion cell potential more likely during local condensation in humid environments. The average size of dew droplet formation on surfaces at different tem-
peratures varies from 20 to 50 µm at about 50 percent relative humidity (RH) to large droplets of water at over 60 percent RH. Add in the natural capillary action under low-profile components with standoff heights of 25 µm and water bridge shorts become a reality. One table salt crystal weighs about 60 µg. This one tiny crystal, spread
thinly enough across a small PCB surface would be enough to fail early stan- dards, such as MIL-P-28809 — greater than 1.56 µg/cm2 or greater than 6.45 µg/in.2. Even invisible amounts of salt (or salt equivalent) left on the board can
present conditions suitable for electrochemical corrosion. Although we under- stand that the salt we are measuring may not be sodium chloride (baseline value), there are various known conversion factors that allow us to convert the baseline values for common PCB assembly contaminants. If there is a reading measurement of 1 µg NaCl/cm2 (salt), but we believe the contaminant to be carbonic acid residue, the equivalent for carbonic acid
would be 0.16 µg H2CO3/cm2, showing that a smaller amount of carbonic acid is more corrosive. Measurements need to be accurate and repeatable. We have the baseline
(salt), but how do we measure accurately with such small trace amounts and do it repeatedly to achieve a Gage R&R of less than 10? A method to achieve this should include: l
Variables with tight controls to measure such minute quantities. l Calibrated solution.
small changes in solution temperature. l
dissolve key components. l
l Temperature compensation, due to the sensitivity of the measurement to Flow rate as fast as practical, but not so fast that it generates turbulence.
February, 2020
l Measurement time of less than three minutes, as longer dwell times can Measurement accuracy of 0.005 µg NaCl/cm2, which is required to achieve
six sigma accuracy and repeatability. The issue with such sensitive measurements is that variables, such as
subtle changes in solution temperature, have an impact on the result. Expo- sure to air, even in the small cavity between the lid and solution can impact final readings. Even more so, high turbulence is experienced, which traps air
in the solution and rapidly dissolves CO2. The solution sounds simple — run the test as fast as possible. However,
run it too fast and bubbles are generated. This impacts the readings as the measurement cell is then measuring air (the bubbles) and not solution con- ductivity. Running it too slow results in extended time in the tank and water absorption into the laminate is enhanced by residues on its surface. Recent studies have shown that solder mask enhances the moisture up-
take of the laminate by four times. This correlates to a study by the European Space Agency, which found that extended exposure in an IPA/DI water solu- tion led to leaching the bromide flame retardant out of the laminate through the solder mask onto the board surface, severely degrading the material prop- erties of the PCB. The analysis of solution with trace levels of ionic matter requires the pre-
cision of a solid gold conductivity measurement cell, within a solid-state meas- uring cell, and connected to a ballistic amplifier. This and a powerful pump allows smooth flow throughout the operation, resulting in a six sigma repeat-
able measurement accuracy down to ±0.005 µg/cm2. Contact: Gen3 Systems, Ltd., Unit B2, Armstrong Mall, Southwood
Business Park, Hampshire, GU14 ONR, UK % +44-125-252-1500 E-mail:
sales@gen3systems.com Web:
www.gen3systems.com r
See at IPC APEX, Booth 1520
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