Plasmas can be considered the fourth state of matter following solid, liquid and gas. These state transformations are caused through the input of energy, which is usually in the form of heat. The input energy in the case of radio frequency (RF) plasma is a rapidly oscillating electromagnetic field between parallel electrode plates. The RF energy is generally applied to the powdered plate, when the opposite plate is typically tied to the ground (Figure 1).
Figure 1. Typical Parallel Plate RF Configuration
Any free electron in the field will follow the oscillation and collide with any matter that crosses its path. When it comes to a gas plasma system, there can be a number of outcomes when energetic electrons collide with gas particles.
The gas particle can become ionized as a result of the addition of electron to the gas particle or more likely through the removal of an electron from the outer orbital of the particle. As a result of the collision, molecules can be cracked apart into reactive radicals, or bump low energy orbital electrons into higher energy orbitals, creating an excited gas particle.
Once this out of place, high energy electron reverts to its lower energy state, the energy loss is compensated by the emission of a photon of light. The conversion efficiency of these excited states is 1% or less, so the overwhelming species inside the plasma is the source species.
The input power from the RF generator, the position in the system relative to the electrodes, and the pressure of the gas in the chamber can manipulate the relative ratios of active species along with the relative energies of those species. The input power drives current flow which produces electron collisions that generate the plasma.
With higher power, more current can be yielded, which yields higher densities of active species. The mean free path of collisions inside the plasma is determined by the gas pressure in the plasma system. There are more collisions at higher pressures, but the mean free path is short so the collisions aren't as energetic. The mean free path is long at lower pressures, where the collision energies are much higher, leading to higher energy species.
The position in the reactor determines how energetic the plasma species is because of the self-induced biases caused from the plasma itself. As the RF energy is capacitively coupled to the electrode, electrons build up on the powered electrode, eliminating any DC ground path. A slight buildup of electrons is also observed on the ground surfaces.
To balance the potentials, the sea of the plasma becomes slightly positive in nature. There is an ion bombardment as a result of these potential differences, as the positive ions from the sea of the plasma are attracted by the negative charge on the surfaces. When the potential difference is greater, the ion energy becomes higher upon impact. Users can treat, clean, or etch the surface of their material properly by taking advantage of these plasma characteristics.
Plasma interacts in two distinct ways with a surface - physically and chemically. The physical interaction happens through ion bombardment of the surface. Energetic ions will impact with the surface, dislodging materials from it. This is a type of sputtering, which is usually conducted with an inert gas like argon.
Chemical interaction with the surface uses active species created within the plasma, including oxygen radicals that are highly reactive with organic materials. It is likely that both mechanisms are present during a plasma treatment, and the dominant mechanism can be controlled through the process parameters such as power, pressure, location, and chemistry.
Due to surface activation of the clean, a plasma treated surface will typically result in high energy surface states. For enhanced bonding, be it lamination bonding, adhesive bonding, or wire bonding, high energy surface states are preferred.
While untreated or low surface energy surfaces usually exhibit hydrophobic characteristics, plasma treated surfaces generally are hydrophilic. Using dyne solutions or measuring the contact angle of a water droplet on the surface can help quantify the wettability of these surfaces.
The contact angles of low energy surfaces will be high and those of high energy surfaces will be low. The water drop contact angle of a polymer before and after plasma treatment can be seen in Figure 2.
Figure 2. Contact angle before and after plasma treatment
In order to modify the surface energy and enhance adhesion using plasma, samples of high performance PCB materials were evaluated. The results of plasma treatment of several commonly used printed circuit board materials are summarized in Table 1.
A Nordson MARCH AP-600 vacuum plasma reactor was used to process each material. The ChemInstruments Tantec Cam-Plus contact angle measuring goniometer was used to measure the contact angles.
The measurement of the contact angles of each material was done before plasma treatment. Plasma treatments that use oxygen chemistry and argon chemistry were performed. After the plasma treatment for each condition, the contact angles were measured.
Samples that were determined to be hard to treat with argon or oxygen were treated using helium, nitrogen, and an 80:20 mixture of hydrogen and nitrogen. These gases have been used in the past for materials like PTFE, which is difficult to treat. Contact angles were again measured before and after each plasma treatment.
The Nordson ASYMTEK SL-940E selective coating dispenser was used to apply conformal coatings to coupons of PCB materials, which were then UV cured or heat cured, depending on the recommendations of the manufacturers. Plasma treated samples and untreated samples were compared, and a Scotch Tape adhesion test was conducted after the cure (Figure 3).
Plasma treatment of enthone SR1000 solder mask coupons was performed at various times and power levels to establish the sensitivity to plasma flux for conformal coating adhesion.1 The adhesion of an untreated sample (a) versus a well treated sample (b) is shown in Figure 4.
The impact of plasma processing time on the adhesion of conformal coating to solder mask coupons can be seen in Figure 5. All process plasma conditions were constant with the plasma exposure time adjusted.
The impact of plasma power on adhesion of conformal coating to solder mask coupons is shown in Figure 6. All plasma process conditions were constant with the plasma RF power input adjusted.
Plasma is excellent for improving adhesion by increasing the surface energy of materials like the preceding solder mask examples. Plasma is also very useful for cleaning contaminants that can adversely affect the adhesion of conformal coatings.2
The effect of plasma clean time on adhesion of conformal coatings to mold release compound contaminated substrates is shown in Figure 7. Oxygen or argon were used to clean the substrates, and the mold release compound can be successfully cleaned from the surface using both chemistries. The oxygen process, which can clean both physically and chemically, is more efficient than the argon process which can only remove the contaminant physically through ion bombardment.
Copper clad high performance materials were obtained from manufacturers and users. The copper was extracted chemically, and after the plasma treatment the materials were evaluated for wettability enhancement. Table 1 shows the summary of contact angle data.
The samples were then evaluated for conformal coating adhesion without any plasma treatment. Both acrylic (Humiseal 1B73) and Polyurethane (Humiseal UV40) conformal coatings were evaluated. In general the epoxy based materials performed well for adhesion without the plasma treatment.
The glass-filled PTFE containing substrates (Rogers 5870) demonstrated more difficulty for adhesion and were in need of more exotic plasma chemistries to improve adhesion. Dupont Pyralux AP-TK, a polyimide material had poor adhesion with no plasma treatment, while the adhesive properties exhibited by Taiyo solder mask was similar to that of the Enthone solder mask.
The glass filled PTFE samples were plasma treated and flown across the country to be conformal coated and cured. There was a delay time of 24 hours, which may of contributed to the noise in the data. Though the sample treated with H2/N2 had the lowest contact angle of the pre-treatments, the best adhesion was exhibited by the sample treated with helium. The comparison of the H2/N2 treated adhesion (a) and the helium treated adhesion (b) is shown in Figure 8.
The Taiyo solder mask sample without plasma treatment can be seen in Figure 9. While the adhesion is poor, the surface wettability is also very poor. The poor wettability can be resolved and the adhesion improved with any form of plasma treatment.
The effects of plasma treatment on the adhesion of conformal coating to polyimide can be seen in Figure 10. While there is a total adhesive failure in the untreated sample (a), excellent adhesion is demonstrated by the plasma treated sample (b)
Conformal coatings for advanced electronic assemblies have numerous challenges that have an effect on the performance. Previously, only military and harsh environment applications required conformal coatings. Today electronics have invaded the consumer handheld space and the resulting hard environments have made conformal coating essential for more assemblies.
Additionally, higher performance materials are required that meet rigid environmental requirements and enable higher frequency applications, such as RoHS.
Plasma surface treatment has proven to be capable of overcoming coating adhesion challenges due to these constraints. The enhanced surface wettability, which is caused by plasma treatment results in enhanced adhesion of conformal coating to high- performance solder mask materials and other substrates that are hard to adhere. PTFE based substrates can also be made more conducive when the plasma chemistry is tailored.
Plasma processing can also remove contaminants on the printed circuit that prevent conformal coating adhesion board. The contaminants can be removed without damaging the substrate, and conformal coating adhesion can be enabled by optimizing the physical and chemical components of the plasma process.
I would like to thank Carla Loeffler of Nordson MARCH for her assistance procuring the PCB materials and Gheorghe Pascu of Nordson ASYMTEK for use of his lab and operating the conformal coater.
 D. Foote, IPC-SMTA High Reliability Cleaning and Conformal Coating Conference, November, 2012
 D. Foote, IPC Electronic Systems Technology Conference, May 2013
This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.
For more information on this source, please visit Nordson MARCH.