Envision HDI is an alternative copper deposition process that just works. Deposition or metalization is the basic foundation of a plated through hole structure. The deposition material makes a drilled hole conductive enough to allow copper to be electroplated within the drilled hole. Without metalization there would be no plated through hole. Of course there are several different chemicals available to metalize a non-conductive surface.
Electroless copper deposition was the original chemical process employed by the pcb industry to form this conductive layer. There were many downsides to this process. Chemical cost, time of operation and waste treatment to name just a few. Some of the chemicals used in the overall process are highly toxic. The process has been improved over the years.
Copper deposition was the only option for a number of years until the introduction of alternative deposition processing. The alternative processing employs either a semi-conductive carbon or graphite. Both require that the material be applied chemically and then baked or cured on. The advantage to these alternatives is the short processing time required. In a world where time is money this is a big advantage. Chemical costs and the load on waste treatment are also less.
The disadvantage to these alternate deposition processes is that they both leave a residue on the outer layer copper surface. This residue shall result in plating nodules and pits if not removed. A micro-etch is employed to remove the residue. For double sided printed circuit boards this is not an issue. For multilayer boards it may be problematic. When the outer layer copper is being micro-etched the inner layer copper at the drilled hole is also being micro-etched at the same time. This introduces the potential of negative etch-back. The amount of negative etch-back is dependent upon the equipment and process used. Reworked process panels are also a factor.
I’ve worked with several versions of metalization. By far the best and most reliable that I have come across is Enthone’s Envision HDI. It is a simple 3 step process that deposits a semi-conductive polymer material within the hole. Ten to one aspect ratio may be achieved in a vertical process line. Twenty to one or better may be achieved in a horizontal process line. This chemistry just works. Envision HDI is also a major source for improved yields. Not a bad problem to have.
There are a few other benefits to Envision HDI at the bare board level. Envision HDI has a low viscosity. This allows for improved wet-ability of the drilled hole. If you can wet the hole you can metalize the hole. Technically it may be considered a green process. There is very little burden on the waste treatment system. The process does not affect the copper and leaves behind no residues. There is no micro-etch required to remove residues that are not left behind as a result of the process. Envision HDI is a purely additive process.
There are two challenges with the process. The first challenges is that the second of the three chemistries operates at a high temperature. The tank, chamber and or sump need to be made of stainless steel or titanium. For a vertical operation this is not a big deal. For a horizontal process this adds to the cost of the one chamber. The second challenge is that the third of the three steps has a limited life span. One week is standard. The next generation of the process chemistry is supposed to include an extended life version of the third step.
A few years ago my team and I converted a vertical OSP line to run the Envision HDI process. Our main focus was to make this as bullet proof as possible. The line has vibration along the entire length. Tank one has ultrasonic vibration. We also rely on a special tool for tank one we lovingly call “VOID BUSTER”. Void Buster is basically a rubber mallet we use to hit the process rack a few times. Yes its primitive and you may chuckle at the thought but it works. Once you wet the hole the process works. Its critical to remove the air from the hole. Using a rubber mallet is not required but its cheap insurance to ensure all air is removed from the hole.
The break down of the process is as follows…
The process begins with our first step, the conditioner. The conditioner wets the hole and promotes the adhesion of the conductive polymer to surfaces difficult to cover. The conditioner applies a thin film on non-metallic surfaces. This step allows the exposed fiberglass to accept the manganese dioxide (MnO2) provided in the second step of our process. It also promotes adhesion of the MnO2 onto other materials such as Teflon and polyimide. Wetting of the hole is very important. It is critical to remove all traces of air from the hole. Vibration, ultrasonics and solution movement provided by side to side agitation or eduction are critical in the first process step. In my experience once the side wall of the hole is coated the rest of the process works. Entrapped air in the hole prevents the film from forming on the side wall of the hole.
After the conditioner step the holes are subjected to a series of rinses. This removes excess chemistry and reduces the amount of chemical drag-out of the first step chemistry into our second process step. The second process step is the initiator. This is a high temperature process step. The initiator coats the non-metallic surfaces in the hole with MnO2. The MnO2 is the conductive portion of the semi-conductive polymer.
After the initiator step the holes are again subjected to a series of rinses. The rinses between the second and third process steps is critical. Drag-out from process step two into process step three should be kept to a minimum. Step three of our process is the catalyst. The catalyst is an organic monomer that combines with the MnO2 and forms the semi-conductive polymer. Process step three is self-limiting. Once all of the MnO2 is combined with the monomer the process stops.
Following the third process step are another series of rinses to remove any excess chemistry. After the rinses an anti-tarnish (Enthone CU-56) may be applied. After the anti-tarnish is applied a final rinse is done followed by drying. Drying is a critical step for it is important to remove any excess moisture from the through holes if Envision HDI is used with a pattern plate operation. The moisture would not damage the semi-conductive polymer. Pattern plating uses a dry film photo resist to act as a plating resist. Moisture in the holes may lock the dry film in at the knee of the hole and prevent that area from plating with copper and then tin. This ultimately results in a rim void. Panel or flash plating is another story. Drying needs to be done to minimize copper oxidation. However, if panel or flash plating is done in line with Envision HDI the holes are already wet and ready for processing.
Reworking of the conductive polymer requires the applied polymer to be completely stripped off. This can be done quickly and efficiently through an existing permanganate desmear process line. The solvent swell is avoided since we do not want to attack the laminate. The permanganate dissolves and removes the semi-conductive polymer when it comes into contact. After the permanganate step the panels should be processed through desmear rinse, neutralizing and final rinse operations.
Envision HDI should be appealing to a designer, equipment manufacturer or contract manufacturer. One of the advantages is that the plated through hole structure is improved. Assembly/board level failures at the hole are minimized. This equates to improved yields not only at the bare board level but also at the assembly level. Holes metalized with Envision HDI have superior IST results compared to other metalization methods. Durability in the field is improved. The semi-conductive polymer also improves CAF resistance since it acts as a barrier at the copper to glass interface.
Envision HDI is also ideal for wire bondable applications. Wire bondable assembly requires a pristine surface to assemble to. The surface mount pad must be perfectly co-planar. A plating nodule or pit are enough to result in an assembly failure. No residues are left behind by Envision HDI that may form a plating nodule or pit. Other metalization techniques leave behind a residue that forms a seed layer that promotes the growth of nodules or formation of pits.
More detailed information on Envision HDI may be downloaded through the following links…
For further information on ENVISION HDI direct metallization system, please contact Enthone Inc. at firstname.lastname@example.org or visit greenmetallization.org.