Hot Air Leveling Alternatives
Rudy Sedlak, RD Chemical Company
How Much Do You Want to Spend?
RD Chemical realized that the PCB fabricators would require a flatter surface than HAL could provide. After some research into several alternatives, the choices became obvious and painful. Essentially the principal options boil down to choosing between precious metal systems that give incredible performance, at a corresponding incredible cost, or organic coatings, that although affordable, do not match the performance level (read solderability) of a Hot Air Leveled surface. There are other interesting options, however these bring with them other complexities, such as a requirement for multiple imaging.
The economics of the intrinsically expensive precious metal coatings are made worse by the fact that they are applied from electroless systems, which bring with them the necessity for periodic replacement, and disposal of heavily chelated chemical systems. Some of the precious metal coatings have an undercoating of electroless Nickel, which doubles the disposal problems. Further since the less expensive Hypophosphite Nickel will not start plating on Copper, a catalyst must be used, which raises the possibility of plating on the soldermask. This is not a pretty picture.
At RD Chemical, it was realized that there was a precedent for a very inexpensive and serviceable precious metal coating already in the industry. Texas Instruments had been applying a very thin, less than 5 millionths inch, coating of Palladium directly on to copper to protect the solderability of component packages that they supply, with great success for a number of years. The key problem with this idea for the PCB fabricators was that the coating applied by TI was electroplated, and this, of course, was not an acceptable process for coating PCB’s pads and holes only.
If the process were to be made acceptable to the PCB fabricators, a non-electrolytic plating process would have to be developed. Non-electrolytic plating processes are normally of two types, either “electroless” or “immersion”. The characteristic that differentiates the two processes, “electroless” and “immersion” has to do with the plating process itself.
When a metal is dissolved in a plating bath, it carries an electrical charge, typically it is a positive charge. In the process of being deposited on the work, or plated, electrons are added to the metal, and the charge is neutralized. In an electroplating bath, the electrons are “pumped” in by the rectifier, which can be referred to as an electron pump.
In an “electroless” plating bath, the electrons are donated by a chemical which is already in the bath, which is called a reducing agent. In an electroless copper bath the formaldehyde is the reducing agent. In Electroless Nickel it is either Hypophosphite, or Dimethyl Amine Borane, the most expensive reducing agent known to the civilized world…….or to the PCB industry.
In an immersion plating bath, the electrons are supplied by the base metal, and effectively the base metal is the reducing agent. And as the base metal donates electrons to the metal being plated, the base metal itself goes into solution, which is why this is also called a replacement reaction. The key characteristics of the immersion plating bath is that it is self limiting, which means that once the base metal, Copper, in the case of a PCB, is covered, the plating ceases. This can result in a very thin, dense, non-porous coating, that is very economical, from a bath that is extremely stable, and impossible to drop out, unless it is desired, for waste treatment.
The operative word above is “can”. It “can” result in these fine characteristics if you put in a year plus of intensive research, that is punctuated with developments that kill the project completely. Whereupon you give the project shock therapy to revive it, more than once, and wind up burning, in one year, your research budget for the next few years, all in the name of the perfect HAL replacement.
RD Chemical Company has developed an immersion Palladium plating bath that puts down a 3 microinch, non-porous coating of Palladium, directly on to copper. Directly on to copper, with nothing under the copper, NOTHING. It is not necessary to underscore the fact that nothing costs just that?
This is all well and good, but immersion baths normally load up with base metal, and require dumping and replacement on a regular basis. This is, of course, complicated by the fact that you never get all the precious metal out of solution, and just to add that special twist, the baths are usually chelated.
One of the serendipitous aspects of the Paladin process bath, is that the bath is a never-dump type, in other words it is treated just like an electroplating bath. This little gem of a feature is achieved because the copper is only very slightly soluble in the bath, and falls out of solution after the bath concentration reaches 100 ppm. and is then filtered out. The total chemical maintenance on the bath is to maintain the metal content, and the pH.
I would like to say that we planned this feature from the start, but I couldn’t say that while looking anybody in the eye. This feature was noticed only after we had solved the other problems inherent in the process.
We are experimenting with a process that will be either two or three steps. The first one or two steps will be the cleaner steps. The final step will be the plating bath. The plating bath is mildly acidic, and is compatible with all solder masks. Processing will be at 120° F, for 3 minutes.
At this point, testing on the coating has been minimal, but promising. Because of setbacks in the development process, we have only had a working bath for a few weeks, and are trying to get up to speed as quickly as possible. We have done some steam aging, as well as multiple passes down an IR soldering oven, and have yet to be able to render the coatings un-solderable. We have done some porosity testing with Nitric Acid, and the results are very promising, as we have seen no porosity.The Paladin process brings the performance of a precious metal coating at a cost that dramatically lower than previously available precious metal plating systems. In fact the cost is so low that it makes it worthwhile to reconsider the cost/performance tradeoffs in the organic systems.