Plasma Treatment Process prior to Conformal Coating
The corrosion of Nickel-Palladium-Gold (Ni-Pd-Au) finish terminals in humid enviroments is known to be reduced with the application of a conformal coating such as acrylic. Corrosion has a higher rate of occurance around the terminal “knee” of a surface mount component, which may be reduced with the application of conformal coatings. Although radio frequency (RF) plasma processing is generally known to enhance conformity of conformal coating to surfaes through iconic bombardment, the effect on the functionality of assembled printed circuit board (PCB) is not as well known.
The purpose of this article is to assess whether RF plasma processing can enhance the adhesive and coverage qualities of an acrylic conformal coating on PCBs, specifically on Ni-Pd-Au terminals with a knee, and if plasma processing has an effect on the electrical functionality of components and fully assembled PCB.
Optical metrology was used to quantify and photograph the enhanced knee surface after boards were plasma processed and conformally coated with acrylic; electrical testing of printed circuit boards was used to determine electrical functionality. Results show that plasma processing can enhance the quality of the conformal coating as well as increase thickness of the coating around the knee of a terminal. Settings of RF power, process gas, process pressure, and plasma process time were used to come to this result.
Terminals finished with Nickel-Palladium-Gold (Ni-Pd-Au), a common terminal plating finish, are known to be susceptible to corrosion – notably in outdoor environment applications. Conformal coatings are well known to mitigrate this corrosion by means of parylene, urethane, epoxy, and acrylic.
There are lots of studies demonstrating the conformity of coverage of the conformal coating to the underlying printed circuit board. Reactive ion bombardment using an argon RF plasma process is one known method to increase surface adhesive by means of the kinetic transfer of atomic energy incident to the surface under bombardment which creates dangling bonds. Oxygen was also considered for simultaneous use with argon because of its capability to remove fluxes and organic compounds found in solder reflow based processes that may not be removed with conventional aqueous wash.
Base on these advantages, plasma processing before application of an acrylic-based conformal coating was proven, with focus on conformity of coverage around the Ni-Pd-Au terminals and the knee of the terminal solder connection of a PCB with multiple components.
Although there is an advantage of plasma processing in these regardsm the posibility that plasma process would affect functionality of components used on the PCB assembly was a concern. The variety of components on the PCB includes discrete components, active components, and programmable microcontrollers. Many studies, one of them was coordinated between Nordson, AirBorn Electronics, and the Desich SMART Center to investigate the viability of the plasma process. And they gave the positive result of plasma’s impact on PCB performance after conformal coating.
Purpose of experiment
The objective of this experiment is to evaluate the effects of RF plasma processing on the conformity of coverage of conformal coating of the knee of individual Ni-Pd-Au leads on electronics assemblies using Humiseal 1B31 Acrylic and to determine if any change in electrical functionalilty occurs. The specific area of interest is the coverage of the coating on the Ni-Pd-Au knee (Figure 1) of a surface mount SOIC20 microcontroller – a programmable microcontroller – on each assembly.
Figure 1: Side profile of SOIC20 microcontroller with knee highlighted in green
The evaluation is accomplished in three stages, with objectives for each below:
Stage 1: test the effect of argon RF plasma process parameters on discrete components in order to mitigate concern of the effect of vacuum pressure and plasma power on electric functionality.
Stage 2: evaluate the effect of plasma power and process gasses on semi-populated PCB using optical metrology and electrical testing on the SOIC20 microcontroller.
Stage 3: evaluate the effect of plasma time and process gas pressure on both semi-populated and fully-populated and functional PCB using eletrical testing and optical metrology.
Conclusion
Processing PCBs in the plasma system increased the conformity of coverage of conformal coating layer to the components on the board and did not affect electrical functionality. It was shown in stage 2 that 225 watts of RF plasma power had as much effect as 300 watts of RF plasma power in regards to improvement of conformal coating. The lower of the two plasma power settings was selected as the optimal setting based on the concept that lower power lessens possible risk of damage to active components on a board and still provides advantages to conformal coating. The lower process pressure of 150 mTorr was selecting as the optimal setting because the higher process pressure is believed to limit the argon plasma capability. With a higher process pressure the mean free path of the charged particles is reduced by the increase of additional gas, which reduces the kinetic effect of bombardment and thus decrease the surface tension at the coating interface. An argon and oxygen blend of 80 percent-20 percent, respectively, was selected as the optimal setting over a solely argon process. Overall the experiment was shown to be successful.
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