Using Plasma Processing to Enhance Conformal Coating

Nickel-palladium-gold (Ni-Pd-Au) is a standard terminal plating finish, and terminals coated with this plating finish are believed to be vulnerable to corrosion, particularly in outdoor environment applications.[1,2] Conformal coatings are highly recognized for reducing this corrosion through parylene,[3] urethane, epoxy, and acrylic.[4]

However, one aspect that was highly prominent in these studies was a reflection on the conformity of coverage of the conformal coating to the underlying printed circuit board (PCB). Typically, reactive ion bombardment using an argon RF plasma process is a well-known technique used for boosting surface adhesion through the kinetic transfer of atomic energy incident to the surface being bombarded, which, in turn, produces dangling bonds.[5] Another element that was also considered for concurrent use with argon was oxygen, as it is capable of removing fluxes and organic compounds present in solder reflow-based processes that otherwise may not be eliminated through a regular aqueous wash.

Based on these benefits, plasma processing before the application of an acrylic-based conformal coating was opted to be studied, together with a focus on the conformity of coverage around the Ni-Pd-Au terminals as well as the knee of the terminal solder connection of a PCB with numerous components.

Although there is a benefit of plasma processing in this regard, the possibility that plasma process would have an impact on the functionality of components employed on the PCB assembly was definitely a major concern. The wide range of components on the PCB includes programmable microcontrollers, discrete components, and active components. In order to look into the feasibility of the RF plasma process, a research analysis was coordinated between Nordson MARCH, Nordson ASYMTEK, the Desich SMART Center (DSC), and AirBorn Electronics.

Objective

This experiment aims to evaluate the impacts of RF plasma processing on the conformity of coverage of conformal coating of the knee of single Ni-Pd-Au leads on electronic assemblies utilizing Humiseal® 1B31 Acrylic, and it also aims to define whether electrical functionality undergoes any particular change. The specific area of interest refers to the coating coverage on the Ni-Pd-Au knee (see Figure 1) of a programmable microcontroller — surface mount SOIC20 microcontroller — on every assembly.

Side profile of SOIC20 microcontroller with knee highlighted in green.

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

The assessment is performed in three stages, with the following goals for each:

  • Stage 1: The impact of argon RF plasma process parameters on separate components should be tested to reduce concerns related to the effect of plasma power and vacuum pressure on electric functionality.
  • Stage 2: The impact of process gases and plasma power on semi-populated PCB should be assessed using optical metrology and electrical testing on the SOIC20 microcontroller.
  • Stage 3: The impact of process gas pressure and plasma time on semi-populated, fully populated, and functional PCBs should be assessed using optical metrology and electrical testing.

Methodology

Separate components were tested in Stage 1 to examine the impacts of plasma process parameters on the same kind of electrolytic capacitors used on the fully populated PCBs. Every capacitor was determined for capacitance and impedance, in addition to being linked to an oscillator circuit that developed a capacitive-based frequency both before and after preliminary pressure and plasma testing. Once samples were exposed to 10 mTorr vacuum pressure, they were run through baseline argon RF plasma processes to ascertain the negligible change in tested electric functionality.

For Stages 2 and 3 of the experiment, four sets of boards were used. Set A includes six semi-populated boards. Every board in Set A (see Figure 2) included a microcontroller and SMT components that can be utilized for powering the microcontroller and switching on an LED. Set B included six PCBs that were populated with only a microcontroller, to be used only for optical measurements (see Figure 3).

Example of a board in Set A.

Figure 2. Example of a board in Set A.

Example of a board in Set B.

Figure 3. Example of a board in Set B.

Set C included six functional and completely populated PCBs (see Figure 4) with SMT components, surface mount components, and discrete components. Set D contained one fully populated PCB (see Figure 4) to be utilized as a control, without RF plasma processing.

Example of a board in Sets C and D shown with permission of AirBorn Electronics.

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

The thicknesses of conformal-coated coupons were determined in Stage 2 to achieve baseline film thickness data and also to establish the viability of optical measurements through the Humiseal® 1B31 acrylic on a flat surface. Preliminary photos of the PCBs were taken to establish the best angle at which to determine the microcontroller’s pins. To measure these pins, stands were built to hold the boards at fixed reference angles. In Stage 2, optical measurements were made at DSC of the microcontroller pins and every board was tested in Set A at AirBorn for electric functionality both before and after plasma processing and conformal coating.

While changing the oxygen/argon gas mixture and plasma power at the time of the plasma process, each PCB was processed. While the conformal coating was done at the Nordson facility, plasma processing was carried out at DSC in an ISO (class 1000) cleanroom. These logistics required the transportation of plasma-processed PCBs in vacuum-sealed N2 purged static shield bags prior to conformal coating. The resulting optimized coverage around the knee of the microcontroller pins was employed for controls in Stage 3.

Stage 3 involved optical measurement of each board in Set B at DSC and then electrical testing of each board in Set C at AirBorn. Due to the restrictions in optical focus as well as the size of the components on the completely populated board, boards in Set C were not optically determined. Using the same plasma settings, a board from Set B was run with a board from Set C. Each pair of boards was processed with varying plasma time as well as backfill gas pressure at the time of the plasma process. After conformal coating, boards in Set C were electrically tested. Similarly, boards in Set B were optically determined after conformal coating.

Experimental Setup

Each separate component was measured with an Agilent E9490A LCR meter to achieve Stage 1 setup. Every component was determined at a fixed 1 kHz frequency and values of capacitance and impedance. An Agilent DSO-X 2024A Oscilloscope and a square wave oscillator circuit in which frequency is controlled by capacitance were also used to test components. Peak-to-peak voltage and increasing overshoot percentage were extra electric values that were determined.

All the discrete components were made to undergo two process parameter tests in a Nordson March AP-300 Plasma System. In the first process parameter test, components were exposed to a vacuum of 10 mTorr for a period of 5 minutes, followed by subjecting the same parts to an argon plasma treatment for 2 minutes at 225 W and with a process pressure of 170 mTorr.

Throughout the experiment, a base pressure of 80 mTorr and a pressure range of 40 mTorr continued to be constant for all plasma processes of boards and components. A solid ground shelf and a solid power shelf were placed in slots 3 and 6, respectively, leaving a gap of 2.75″ between the shelves. Both before and after all these tests, the parts were tested. In order to detect potential variations related to the test equipment or cleanroom environment, a reference capacitor was tested in all stages.

Stage 2 involved the initial testing of boards in Set A for electrical functionality at AirBorn. These boards were first brought to DSC in static shield bags, cleaned according to cleanroom protocols, and optically determined using a Lumenera Infinity 1 camera and a Nikon Measurescope MM-400. Images of the microcontroller’s pins 10, 11, 13, 15, and 17 were taken on the z-axis. Then, using the NIS-Elements D software, the focused sections of each photograph of one pin were integrated to produce a single image of the fully focused pin, which was utilized for metrology, as illustrated in Figure 5 for pin 10.

Example of a focused image of pin 10 of uncoated microcontroller pin with measurement reference indicated.

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

The Nordson MARCH AP-300 plasma system was programmed to vary process gas ratio and plasma power between recipes and, at the same time, keep process pressure and process time 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 W, process gases remained constant at 80% Ar/20% O2.

Process Pressure Process Time
mTorr seconds
150 500 60 120 180

In Set A, boards were separately placed in the plasma system, run at the chosen recipe, purged, closed in an antistatic bag, and finally delivered to Nordson facilities. Within one hour of plasma treatment, each board was conformally coated on a specific section. To complete the coating, a SL-940E with a Viscosity Control System was used to dispense Humiseal® 1B31 Acrylic mixed with a 2:1 Xylene to Acrylic mixture and then dispensed with an SC-280C circulating design dispensing head. Afterward, boards were run through a TCM-2200 curing oven for a period of five minutes, using settings of 25% convection heat and 75% infrared, and this was followed by a room temperature cure for 24 hours.

Then, boards were electrically tested for microcontroller functionality after the completed cure of the conformal coating of boards in Set A. This was done using Airborn-established methods and the boards were also optically determined using the Nikon measurescope at DSC. Measurements, which were mainly concentrated around the knee of the pins on the microcontroller, were taken both before and after the plasma and coating processes. These measurements were then compared to ascertain the plasma process parameters for Stage 3.

In Stage 3 arrangement, the pins of the microcontroller need to be measured for boards in Set B and the plasma process parameters need to be established for boards in Sets B and C. In accordance with cleanroom protocols, boards in Set B were first cleaned and then optically examined and determined by applying the aforesaid process using the measurescope. The plasma system was designed to vary process time and process pressure between recipes and, at the same time, keep process gas ratio and plasma power constant, as shown in Table 1.

From the Set A group, a single recipe was repeated for verification purposes. The boards from Set B and Set C were first placed in the plasma system in pairs, processed simultaneously with a chosen recipe, purged, enclosed in individual anti-static bags with the help of an Accu-Seal Model 35-23G vacu-purge sealer, and finally delivered to Nordson facilities. The same equipment and parameters employed for boards in Set A were applied to conformally coat each board within an hour of plasma treatment. The control board in Set D was transported and coated with boards in Set B and Set C.

Then, following the total cure of the conformal coating of boards in Sets B and C, the boards in Sets C and D were subjected to full functionality testing using Airborn-established techniques and the boards in Set B were optically measured using the Nikon measurescope at DSC. Measurements of the pins both before and after the plasma and coating processes were compared to ascertain the optimal plasma system parameters for conformal coating.

Results

Testing in Stage 1 showed below 2% difference in the electrical behavior of all electrolytic capacitors pre and post vacuum processing and plasma processing. This difference was within acceptable levels.

For Stage 2, optical metrology displayed a total of 40–60 μm of 1B31 acrylic coated on the pins of boards of Set A. Moreover, the uncoated microcontroller Ni-Pd-Au terminals determined a knee thickness ranging between 220 and 230 μm at measurement point indicated in Figure 5. Pre- and post-coating processes were optically measured, which demonstrated an increased thickness of attached acrylic utilizing an RF plasma power level of 225 W, with an 80%:20% mixture of argon (Ar) to oxygen (O2) gas, respectively. When the mixture of Ar/O2 was applied, an increased thickness of 275–285 μm was measured at the knee.

This increase in thickness was 15% greater when compared to the parts processed in Ar plasma alone. Visual observations showed that the acrylic coating had better coverage, especially beneath the pin and on the lower part of the microcontroller as shown in Figures 6 and 7.

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

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

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

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

When O2 was left out of the plasma, “stringers” and bubbles are often formed between the SOIC20 package and the pin, as illustrated in Figures 8 and 9.

Board Set B before being processed in Ar plasma.

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

Board Set B after being processed in Ar plasma.

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

The coating thickness on the pins following the use of the Ar/O2 mixture was found to be the same between the 225 W and 300 W plasma. Since both thicknesses were 5% thicker than the 150 W plasma, the 225 W plasma was eventually selected that has an 80%:20% mixture of Ar to O2 gas, respectively. Electrically tested boards in Stage 2 appeared to be practical. Every microcontroller was able to receive power, output the programed oscillation, and take in new code.

With regards to Stage 3, optical metrology showed a total of 40–60 μm of 1B31 acrylic coated on the boards of Set B. Upon increasing the Ar/O2 process pressure from 150 to 500 mTorr, the knee on the pin remained suitably coated; however, additional stringers and bubbles were seen under the pin and also around the bottom of the microcontroller, as shown in Figures 10 and 11.

Before the board was processed in 500 mTorr process pressure.

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

After the board was processed in 500 mTorr process pressure.

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

The bubbles that resulted after processing in 500 mTorr pressure made it difficult to make a direct measurement of thickness at the knee, but it is expected to be between 240 and 260 μm.

Using the same process parameters in Stage 2, a single board was processed and repeated in the thickness range of 275–285 μm around the knee. When the processing time was increased, it made a positive impact by increasing the material’s thickness from the ensuing settings in Stage 2 by as much as 3%, especially around the knee as can be seen in Figures 12 and 13.

Before the board was processed in optimal settings.

Figure 12. Before the board was processed in optimal settings.

After the board was processed in optimal settings.

Figure 13. After the board was processed in optimal settings.

The optimal settings for this outcome are shown in Table 4, which include a range of 40 mTorr, a base pressure of 80 mTorr, and a powered solid RF shelf setting with one 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

This article showed how the use of Nordson MARCH AP-300 plasma system for PCB processing successfully increased the conformity of coverage of the Humiseal® 1B31 acrylic to the components on the board and did not have an effect on electrical functionality. It was shown in Stage 2 that 225 W of RF plasma power had an equal impact as 300 W of RF plasma power with regards to the improvement of conformal coating. The lower of the two plasma power settings was selected as the optimal setting on the basis of the concept that lower power reduces the potential risk of damage caused to active components on a board and still continues to be advantageous to conformal coating.

The lower process pressure of 150 mTorr was chosen as the ideal setting because it is believed that higher process pressure limits the capability of argon plasma. Due to a higher process pressure, the charged particles’ mean free path is decreased by the increase of additional gas, which, in turn, reduces the kinetic effect of bombardment, thus reducing the surface tension at the coating interface. An oxygen and argon mixture of 20% and 80%, respectively, was selected as the optimal setting over an exclusive argon process. On the whole, the experiment was shown 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 thank 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 article. 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 article.

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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