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October, 2019 High-Viscosity Fluid Air Assist Dispensing By James Hall, Senior Engineer — Manufacturing, BCS Automotive Interface Solutions B
CS Automotive Interface Solutions, located in Auburn, New York, man- ufactures automotive tire pressure monitoring sensors. The demand for more rigid tire pressure monitoring (TPM) sensors are on the rise to
comply with greater operating demands for increased tire pressure loading. BCS requires the ability to inject a high-viscosity potting compound into a heavy-duty sensor. The design of this assembly imposed some design
challenges in how the material flows through narrow shallow housing beneath a PCB assembly. Production had been reporting 0.36 percent scrap rate for potting fill issues. This initiated continuous improvement activities that entailed incorporating the function of air assist dur- ing dispense. BCS then established a strong partnership with Precision Valve Automotive (PVA) of Cohoes, New York, in providing its technical services.
Process Development The process that was developed required extensive
research and development. The dispense process was first conceptualized at DFM, which involved teaming up with PVA’s technical support inside its R&D department. The Delta-8, along with its many available options, provided the greatest flexibility in meeting BCS’ process require- ments. The VPX-2K integrated volumetric pump system of-
fered BCS the best dual-head dispense solution for a two- component mix ratio. The high precision, rotating posi- tive displacement pump works extremely well when dis- pensing low-volume shot sizes that involve a high-viscos- ity material. To maintain dispense precision for low-vol- ume shot sizes, a high-precision digital scale was incorpo- rated into the system with resolution down to 0.1 mg. This scale is a must for in-process volume monitor-
ing and calibration controls when dealing with very low- precision shot sizes. To ensure dispense accuracy, trace- ability of the material flow (monitoring) is equally as im- portant to prepare the proper mix ratio for each assembly.
A cross section of PVA’s VPX-2K dispensing pump.
The technical challenges for dispensing a high-viscosity material in low
volume through narrow channels underneath a PCBA required both addition- al heating and part tilt to compensate for the high wetting properties (capil- lary forces) of the material being dispensed inside the TPM sensor. Once the process was released in high volume, manufacturing started re-
porting a 0.36 percent fallout for potting not flowing completely out around the housing well, even though all process conditions and controls were found to be within process specifications. During failure analysis, all suspect parts displayed a unique common signature that suggested a plausible the- ory for root cause.
Failure Analysis The theory suggested that during initial dispensing,
any material that swells and touches the backside of the PCB fill hole has enough wetting force to freeze in location before having the ability to flow out in the potted well. This was noted in earlier development but was thought to be controlled by the dispense programs — shot volume, heat, pump speed, seating angle, and fill time. Unfortunately, production had to run thousands of
TPM sensors to understand the variability of the phenom- ena. Pump speed and fill time were key factors in control- ling contact around the PCB’s fill hole. The VPX-2K pumps and fill time had to run slower given the con- straints of the material’s viscosity, which resulted in sac- rificing cycle time. The strength of the material is its wetting forces
properties. Unfortunately, these excellent wetting forces can impede the potting process. If any of the material touches the underside of the PCB fill hole regions it will attempt to form a web and render its inability for the ma- terial to advance into the well. The material acts much like glue stringing between
two adjacent surfaces. PVA already had a blow-off feature on its stop-cock mixer tube assembly that was used to
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