Plasmas can be regarded as the fourth state of matter after solid, liquid, and gas. These state conversions are brought about by the input of energy, which is generally in the form of heat. A quickly oscillating electromagnetic field between parallel electrode plates is the input energy for the radio frequency (RF) plasma. The RF energy is usually applied to the grounded plate, when the other plate is normally fastened to the ground (Figure 1).
Figure 1. Typical parallel plate RF configuration.
Any free electron in the field tends to follow the oscillation and bump into any matter that comes across its way. In the case of a gas plasma system, several results can be obtained when energetic electrons bump into gas particles.
The gas particle can be ionized due to the inclusion of electron into the gas particle or more likely through the removal of an electron from the outer orbital of the particle. Due to the collision, molecules can be broken apart into reactive radicals, or shift low energy orbital electrons into higher energy orbitals, forming an excited gas particle.
Once this excited high-energy electron returns back to its lower energy state, the energy loss is balanced by the release of a photon of light. The conversion efficiency of these excited states is less than or equal to 1%; therefore, the overpowering species within the plasma is the source species.
The relative ratios of active species together with the relative energies of those species can be controlled by the input power from the RF generator, the pressure of the gas in the chamber, and the position in the system in relation to the electrodes. The input power induces current flow, which results in electron collisions that produce the plasma.
As the power is increased, the current yield will be more, which produces higher densities of active species. The gas pressure in the plasma system determines the mean free path of collisions within the plasma. At higher pressures, there are more collisions; however, as the mean free path is short, the collisions are not so energetic. At lower pressures, the mean free path is long and the collision energies are much higher, resulting in higher energy species.
The position in the reactor decides how energetic the plasma species are due to the self-induced differences produced by the plasma itself. Since the RF energy is capacitively coupled to the electrode, electrons accumulate on the powered electrode, removing any DC ground path. Furthermore, a slight accumulation of electrons is noticed on the ground surfaces.
In order to maintain the potentials in equilibrium, the sea of the plasma becomes somewhat positive in nature. Due to these potential differences, there is an ion bombardment since the positive ions from the sea of the plasma are pulled toward the negative charge on the surfaces. As the potential difference increases, the ion energy is also increased upon collision. Users can treat, wash, or etch on the surface of their material appropriately by leveraging these plasma characteristics.
Plasma interacts with a surface in two different ways — chemically and physically. The physical interaction takes place via ion bombardment of the surface. Energetic ions will collide with the surface, dislocating materials from it. This is a kind of sputtering, which often happens with an inert gas such as argon.
Chemical interaction with the surface employs active species formed inside the plasma, for example, oxygen radicals that are highly reactive with organic materials. Both mechanisms can be present during a plasma treatment, and it is possible to manipulate the dominant mechanism via the process parameters such as pressure, power, chemistry, and location.
Surface activation of the clean, plasma-treated surface will generally lead to high energy surface states. Regardless of whether it is adhesive bonding, lamination bonding, or wire bonding, high energy surface states are preferred for improved bonding.
Although untreated surfaces or surfaces with low surface energy normally show hydrophobic properties, plasma-treated surfaces are usually hydrophilic. The wettability of these surfaces can be measured using dyne solutions or by determining the contact angle of a water droplet on the surface.
The contact angles of high energy surfaces will be low and those of low energy surfaces will be high. Figure 2 illustrates the water drop contact angle of a polymer before and after plasma treatment.
Figure 2. Contact angle before and after plasma treatment.
In order to alter the surface energy and improve adhesion using plasma, samples of high-performance PCB materials were assessed. The outcomes of plasma treatment of many widely used printed circuit board materials are given in Table 1 in Figure 3.
Each material was processed using a Nordson MARCH AP-600 vacuum plasma reactor. The contact angles were measured using the ChemInstruments Tantec Cam-Plus contact angle measuring goniometer.
Before plasma treatment, the contact angles of each material were measured. Plasma treatments that use oxygen chemistry and argon chemistry were carried out. The contact angles were again measured following the plasma treatment for each condition.
Samples that were found to be difficult to treat with oxygen or argon were treated using nitrogen, helium, and a mixture of hydrogen and nitrogen in 80:20 ratio. Earlier, these gases have been used for materials such as PTFE, which is hard to treat. Again, the contact angles were measured before and after each plasma treatment.
The Nordson ASYMTEK SL-940E selective coating dispenser was employed to apply conformal coatings to coupons of PCB materials, which were subsequently UV cured or heat cured, as recommended by the manufacturers. A comparison was done between plasma-treated samples and untreated samples, and a Scotch Tape adhesion test was carried out after the cure (Figure 3).
Plasma treatment of enthone SR1000 solder mask coupons was conducted at different times and power levels to determine the sensitivity to plasma flux for conformal coating adhesion.1 Figure 4 illustrates the adhesion of an untreated sample (a) versus a well-treated sample (b).
The effect of plasma processing time on the adhesion of conformal coating to solder mask coupons is shown in Figure 5. All process plasma conditions were constant when the plasma exposure time was fine-tuned.
Illustrated in Figure 6 is the effect that plasma power has on the adhesion of conformal coating to solder mask coupons. All plasma process conditions were constant when the plasma RF power input was fine-tuned.
Plasma is ideal for enhancing adhesion by increasing the surface energy of materials such as the solder mask examples mentioned earlier. Moreover, plasma is very helpful for removing contaminants that can have a negative impact on the adhesion of conformal coatings.2
Figure 7 illustrates the impact that plasma clean time has on adhesion of conformal coatings to mold release compound contaminated substrates. The substrates were cleaned using oxygen or argon, and the mold release compound can be effectively cleaned from the surface using both chemistries. The oxygen process, which can remove contaminants physically as well as chemically, is more efficient when compared to the argon process that can only clean physically by means of ion bombardment.
Copper-clad high-performance materials were received from users and manufacturers. The copper was obtained chemically, and the materials were assessed for wettability enhancement following the plasma treatment. The overview of contact angle data is provided in Table 1.
After that, the samples were assessed for conformal coating adhesion without any plasma treatment. Both polyurethane (Humiseal UV40) and acrylic (Humiseal 1B73) conformal coatings were assessed. On the whole, the epoxy-based materials worked well for adhesion without the plasma treatment.
The glass-filled PTFE-containing substrates (Rogers 5870) exhibited more complexity for adhesion and more exotic plasma chemistries were required to enhance adhesion. Dupont Pyralux AP-TK, a polyimide material, exhibited poor adhesion without plasma treatment, while the adhesive properties displayed by Taiyo solder mask were analogous to that of the Enthone solder mask.
The glass-filled PTFE samples were plasma treated and transported across the country to be conformal coated and cured. There was a delay time of 24 hours, which may contribute to the noise in the data. The sample treated with H2/N2 had the lowest contact angle of the pre-treatment; however, the best adhesion was demonstrated by the sample treated with helium. Figure 8 illustrates the comparison of the H2/N2-treated adhesion (a) and the helium-treated adhesion (b).
Figure 9 illustrates the Taiyo solder mask sample without plasma treatment. As the adhesion is poor, the surface wettability is also extremely poor. It is possible to overcome the poor wettability and to enhance the adhesion using any form of plasma treatment.
Illustrated in Figure 10 are the impacts that plasma treatment has on the adhesion of conformal coating to polyimide. Although there is a total adhesive failure in the untreated sample (a), excellent adhesion is exhibited by the plasma-treated sample (b).
Conformal coatings for sophisticated electronic assemblies face several issues that have an impact on the performance. Earlier, only military and adverse environment applications needed conformal coatings. At present, electronics have occupied the consumer handheld space and the resulting adverse environments have made conformal coating important for more assemblies.
Furthermore, higher performance materials are needed that fulfill strict environmental demands and allow higher frequency applications, for example, RoHS.
Plasma surface treatment has been established to have the ability to resolve coating adhesion issues due to these limitations. The improved surface wettability, which is brought about by plasma treatment, leads to improved adhesion of conformal coating to high-performance solder mask materials and other substrates that are difficult to adhere. Substrates based on PTFE can also be made more conducive when the plasma chemistry is customized.
Furthermore, plasma processing can eliminate contaminants on the printed circuit that prevent adhesion of the conformal coating to the board. The contaminants can be eliminated without causing any damage to the substrate, and conformal coating adhesion can be facilitated by optimizing the chemical and physical components of the plasma process.
 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.