Enhancing the Quality of Conformal Coating with Plasma Processing

Terminals finished with Nickel-Palladium-Gold (Ni-Pd-Au), a common terminal plating finish, are considered to be susceptible to corrosion - especially in outdoor environment applications.[1,2] Conformal coatings are much acknowledged for mitigating this corrosion by means of parylene[3], epoxy, urethane and acrylic.[4] Most noted in these studies, however, was a reflection on the conformity of coverage of the conformal coating to the underlying printed circuit board. Reactive ion bombardment employing an argon RF plasma process is one familiar method for increasing surface adhesion by means of the kinetic transfer of atomic energy incident to the surface under bombardment which creates dangling bonds.[5] Oxygen was also considered for simultaneous use with argon due to its potential for removing organic compounds and fluxes present in solder reflow based processes that may not be removed with standard aqueous wash.

Plasma processing prior to application of an acrylic-based conformal coating was chosen to be studied, based on these advantages, along with concentration on conformity of coverage around the Ni-Pd-Au terminals and the knee of the terminal solder connection of a PCB with multiple components.

Even though there is an advantage of plasma processing in these regards, the likelihood that plasma process would affect functionality of components used on the PCB assembly was indeed a concern. The variety of components on the PCB comprises of active components, discrete components and programmable microcontrollers. In order to examine the viability of the RF plasma process, a study was coordinated between Nordson ASYMTEK, Nordson MARCH, AirBorn Electronics and the Desich SMART Center (DSC).

Objective

The aim of this experiment is to assess the effects of RF plasma processing on the conformity of coverage of conformal coating of the knee of individual Ni-Pd-Au leads on electronic assemblies using Humiseal® 1B31 Acrylic and to define if any change in electrical functionality takes place. The particular area of interest refers to 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.

SOIC20 microcontroller

Figure 1. Side profile of SOIC20 microcontroller with knee highlighted in green.

The evaluation is carried out in three stages, with the below objectives for each:

  • Stage 1: Test the effect of argon RF plasma process parameters on discrete components to mitigate concern of the effect of vacuum pressure and plasma power on electric functionality.
  • Stage 2: Assess the effect of plasma power and process gasses on semi-populated PCB using electrical testing and optical metrology on the SOIC20 microcontroller.
  • Stage 3: Assess the effect of plasma time and process gas pressure on both fully-populated and semi-populated and functional PCB using electrical testing and optical metrology.

Methodology

In Stage 1, discrete components were tested in order to assess the effects of plasma process parameters on the same type of electrolytic capacitors employed on the completely populated boards. Each capacitor was measured for impedance, and capacitance, besides being connected to an oscillator circuit that developed a capacitive based frequency before and after initial pressure and plasma testing. Samples were subjected to 10 mTorr vacuum pressure and run through baseline argon RF plasma processes in order to establish minimal change in tested electric functionality.

Four sets of boards were employed for Stages 2 and 3 of the experiment. Set A comprised of six semi-populated boards. Each board in Set A (Figure 2) had microcontroller and SMT components that could be used for powering the microcontroller and turning on an LED. Set B comprises of six boards populated with just a microcontroller, to be used for optical measurements only (Figure 3). Set C comprised of six completely populated and functional boards (Figure 4) with discrete components, surface mount components, and SMT components. Set D comprised of a single fully populated board (Figure 4) to be used as a control, with no RF plasma processing.

In Stage 2, thicknesses of conformal coated coupons were measured to attain baseline film thickness data, and to determine the feasibility of optical measurements via the Humiseal® 1B31 acrylic on a flat surface. Preliminary photographs of boards were taken in order to determine the best angle at which to measure pins of the microcontroller. Stands were constructed for holding the boards at fixed reference angles for measuring these pins. Stage 2 comprised of making optical measurements at DSC of the microcontroller pins and testing each board in Set A at AirBorn for electric functionality before and after conformal coating and plasma processing.

Each board was processed while changing plasma power and the oxygen/argon gas mixture during plasma process. Plasma processing took place at DSC in an ISO (class 1000) cleanroom; the conformal coating took place at the Nordson facility. These logistics needed plasma-processed boards to be transported in vacuum sealed N2 purged static shield bags before conformal coating. The resulting best coverage around the knee of the microcontroller pins was used for controls in Stage 3.

Stage 3 dealt with optically measuring each board in Set B at DSC and then electrically testing each board in Set C at AirBorn. Boards in Set C were not optically measured because of limitations in optical focus and size of the components on the completely populated board. A board from Set B was run with a board from Set C with the same plasma settings. Each board pair was processed with changing plasma time and backfill gas pressure during the plasma process. Boards in Set C were electrically tested following conformal coating. After conformal coating, boards in Set B were optically measured.

Example of Board in Set A.

Figure 2. Example of Board in Set A.

Example of Board in Set B.

Figure 3. Example of Board in Set B.

Example of Board in Sets C and D

Figure 4. Example of Board in Sets C and D shown with permission of AirBorn Electronics.

Experimental Setup

Stage 1 setup was attained by measuring each discrete component with an Agilent E9490A LCR meter. Each component was measured at a fixed 1 kHz frequency and values of impedance and capacitance. Components were also tested with the help of an Agilent DSO-X 2024A Oscilloscope and a squarewave oscillator circuit in which capacitance controls frequency. Additional electric values measured were increasing overshoot percentage and peak-to-peak voltage.

All discrete components were subjected to two process parameter tests in a Nordson March AP-300 Plasma System. In the first test, components were subjected to vacuum of 10 mTorr for 5 minutes. The same parts were then subjected to a 2 minute Argon plasma treatment at 225 watts with a process pressure of 170 mTorr. A pressure range of 40 mTorr and a base pressure of 80 mTorr remained constant for all plasma processes of components and boards all through the experiment. A solid power shelf was placed in slot 6 and a solid ground shelf was placed in slot 3, leaving a 2.75” space between shelves. Parts were tested both before and after each of these tests. A reference capacitor was tested in all stages to detect possible differences associated with the cleanroom environment or test equipment.

For Stage 2, boards in Set A were initially tested for electrical functionality at AirBorn. The boards were brought to DSC in static shield bags, cleaned based on cleanroom protocols, and optically measured with the help of a Nikon Measurescope MM-400 and a Lumenera Infinity 1 camera. Photographs of pins 10, 11, 13, 15, and 17 of the microcontroller were taken on the z-axis. With NIS-Elements D software, the focused sections of individual photographs for a single pin were incorporated in order to create one image of the completely focused pin, which was used for metrology, as depicted in Figure 5 for Pin 10.

Pin 10 of uncoated microcontroller pin

Figure 5. Example of focused image of Pin 10 of uncoated microcontroller pin with measurement reference indicated.

The Nordson MARCH AP-300 plasma system was programed to differ plasma power and process gas ratio between recipes, while keeping process time and process pressure constant as illustrated in Tables 1, 2 and 3.

Table 1. Nordson March 300 settings for discrete components tested in Stage 1 - gray areas held constant.

RF POWER Process Pressure Process Gasses Process Time
Watts mTorr sccm seconds
0 10 0 300
225 170 100% Ar 120

Table 2. Nordson MARCH AP-300 settings for boards from Set A tested in Stage 2 – process pressure remained constant at 150 mTorr, process time remained constant at 120 seconds.

RF POWER Process Gasses
Watts sccm
150 225 300 100% Ar 80% Ar/20% O2

Table 3. Nordson MARCH AP-300 settings for boards from Sets B and C tested in Stage 3 – RF POWER remained constant at 225 Watts, process gasses remained constant at 80% Ar/20% O2.

Process Pressure Process Time
mTorr seconds
150 500 60 120 180

Boards in Set A were independently positioned in the plasma system, run at the designated recipe, purged and sealed in an antistatic bag, and sent to Nordson facilities. Each board was conformally coated on a particular section within one hour of plasma treatment. Coating was completed using an SL-940E with a Viscosity Control System to dispense Humiseal® 1B31 Acrylic blended with a 2:1 Xylene to Acrylic mixture and dispensed with an SC-280C circulating design dispensing head. Boards were then run via a TCM-2200 curing oven for five minutes, with settings of 75% infrared and 25% convection heat, which is followed by a room temperature cure of 24 hours.

Following the completed cure of the conformal coating of boards in Set A, boards were electrically tested for microcontroller functionality using techniques established by Airborn and optically measured using the Nikon measurescope at DSC. Measurements focused mainly around the knee of the pins on the microcontroller were taken pre- and post-plasma and coating processes. They were compared to establish the plasma process parameters for Stage 3.

Stage 3 arrangement required measuring the pins of the microcontroller on boards in Set B and establishing the plasma process parameters for boards in Sets B and C. Boards in Set B were cleaned based on cleanroom protocols, and optically inspected and measured implementing the aforementioned procedure using the measurescope. The plasma system was engineered to vary process pressure and process time between recipes, while maintaining plasma power and process gas ratio constant as illustrated in Table 1.

One recipe from the Set A group was repeated as verification. A board from Set B and a board from Set C were positioned in the plasma system in pairs, processed at the same time with a designated recipe, purged and sealed in separate antistatic bags using an Accu-Seal Model 35-23G vacu-purge sealer, and sent to Nordson facilities. Each board was conformally coated within one hour of plasma treatment with the same parameters and equipment used for boards in Set A. The Set D control board was transported and coated with boards in Sets B and C.

Next, the total cure of the conformal coating of boards in Sets B, C, and D, full functionality testing using techniques established by Airborn was performed on boards in sets C and D, and optical measurements using the Nikon measurescope were performed on boards in set B at DSC. Measurements of the pins pre- and post-plasma and coating processes were compared to establish the optimal plasma system parameters for conformal coating.

Results

Testing in Stage 1 revealed less than 2% variation in the electrical behavior of all electrolytic capacitors before and after plasma processing and vacuum processing, which was within satisfactory levels.

Optical metrology for Stage 2 exhibited a total of 40 μm - 60 μm of 1B31 acrylic coated on the pins of boards of Set A. The uncoated microcontroller Ni-Pd-Au terminals measured a knee thickness ranging from 220 μm – 230 μm at measurement point referenced in Figure 5. The optical measurements of pre- and post-coating showed an increase thickness of attached acrylic using an RF plasma power level of 225 W, with an 80%-20% mixture of argon to oxygen gas, respectively. When Ar/O2 was applied, there was an increase in thickness of 275 μm – 285 μm measured at the knee.

This increased thickness was 15% greater than parts plasma processed in only Ar plasma. Visual observations revealed that the acrylic coating had improved coverage, specifically under the pin and on the lower portion of the microcontroller as illustrated in Figures 6 and 7. When O2 was left out of the plasma, bubbles and ‘stringers’ frequently formed between the pin and the SOIC20 package which can be observed in Figures 8 and 9.

The coating thickness on the pins after using the Ar/O2 mix was the same between the 225 W and 300 W plasma. Both thicknesses were 5% thicker than the 150 W plasma; thus, 225 W plasma was chosen with an 80%-20% mixture of Ar to O2 gas. Boards that were electrically tested in stage 2 were found to be practical. Each microcontroller received power, output the programed oscillation, and could take in new code.

Board Set A before processed in Ar/ O2 mixed plasma.

Figure 6. Board Set A before processed in Ar/ O2 mixed plasma.

Board Set B after processed in Ar/ O2 mixed plasma.

Figure 7. Board Set B after processed in Ar/ O2 mixed plasma.

Optical metrology for Stage 3 revealed a total of 40 μm - 60 μm of 1B31 acrylic coated on the boards of Set B. When the Ar/O2 process pressure was raised from 150 mTorr to 500 mTorr, the knee on the pin remains properly coated; extra bubbles and stringers, however, are visible under the pin and around the base of the microcontroller as illustrated in Figures 10 and 11. The bubbles that formed after processing in 500 mTorr pressure caused trouble in making a direct measurement of thickness at the knee; however, it is estimated to be between 240 μm and 260 μm.

Board Set B before processed in Ar plasma.

Figure 8. Board Set B before processed in Ar plasma.

Board Set B after processed in Ar plasma.

Figure 9. Board Set B after processed in Ar plasma.

Before board processed in 500 mTorr process pressure.

Figure 10. Before board processed in 500 mTorr process pressure.

After board processed in 500 mTorr process pressure.

Figure 11. After board processed in 500 mTorr process pressure.

Before board processed in optimal settings.

Figure 12. Before board processed in optimal settings.

After board processed in optimal settings.

Figure 13. After board processed in optimal settings.

One board was processed using the same process parameters in stage 2, and repeated in the thickness range of 275 μm – 285 μm around the knee. Expanding process time had a positive effect increasing the thickness of material from the resulting settings in stage 2 by 3%, particularly around the knee as can be observed in Figures 12 and 13. The optimal settings for this result are illustrated in Table 4, which include a base pressure of 80 mTorr, range of 40 mTorr, a powered solid RF shelf setting with single slot space (2.75” clearance) between shelves.

Table 4. Optimal plasma process settings for Humiseal® 1B31 Acrylic on PCB boards and components using the Nordson March 300 Plasma System prior to coating.

RF Power Process Pressure Process Gasses Process Time
225 Watts 150 mTorr 80% Ar/20% O2 180 sec

Conclusion

Processing PCBs using the Nordson MARCH AP-300 plasma system boosted the conformity of coverage of the Humiseal® 1B31 acrylic to the components on the board and did not impact electrical functionality. It was illustrated in stage 2 that 225 W of RF plasma power had as much effect as 300 W of RF plasma power with respect to enhancement of conformal coating. The lower of the two plasma power settings was chosen as the ideal setting based on the concept that lower power diminishes possible risk of damage to active components on a board and still provides benefits to conformal coating.

The lower process pressure of 150 mTorr was selected as the optimal setting because higher process pressure is thought to restrict the argon plasma capability. With a higher process pressure, the mean free path of the charged particles is decreased by the increase of extra gas, which decreases the kinetic effect of bombardment, thereby decreasing the surface tension at the coating interface. An argon and oxygen mixture of 80%-20%, respectively, was chosen as the ideal setting over a solely argon process. In general, the experiment was demonstrated to be successful.

References

1. M. Osterman, (2015) “Effectiveness of Conformal Coat to Prevent Corrosion of Nickel-palladium-gold finished Terminals” IPC APEX EXPO 2014

2. S. Zhan, M. Azarian, M. Pecht, (2006) “Surface Insulation Resistance of Conformally Coated Printed Circuit Boards Processed with No-Clean Flux”, IEEE Transactions on Electronics Packaging Manufacturing, Vol. 29, No. 3, pp. 217-233

3. K. Zhang, M. Pecht,(2000) “Effectiveness of Conformal Coatings on a PBGA Subjected to Unbiased High Humidity, High Temperature Tests”, Microelectronics International, Vol. 17, No. 3, pp. 16-20

4. A. Salman, Z. Burhanudin, N. Hamid (2010) “Effects of Conformal Coatings on the Corrosion Rate of PCB-based Multielectrode-Array-Sensor”, International Conference on Intelligent and Advanced Systems

5. B. Welt (2009) “Technical Synopsis of Plasma Surface Treatments”, University of Florida

Acknowledgements

The authors would like to give thanks to Desich SMART Center members Nate Annable, Matt Apanius, Daniel Ereditario, Mara Rice, and Ben Smith as well as Gheorghe Pascu, Ken Heyde, and Jim Nielsen of Nordson ASYMTEK for the time and effort spent on running equipment and gathering data for this report. Lastly the authors would like to thank Thomas J P Petcavage of AirBorn Electronics for the conjoined work effort in providing work materials and performing testing on all plasma treated and conformal coated parts presented in this report.

This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.

For more information on this source, please visit Nordson MARCH.

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