“Preparation” (Cleaning) of Innerlayers for Photoresist

The whole world seems to be trying to avoid the use of chemicals, and nowhere is that more evident than in the preparation of innerlayers for photoresist. However it is important that we realize that chemicals were, and are, used for one very important reason, when formulated and used properly, they work very well.

There are two issues that need discussing when examining the various methods of innerlayer preparation, the effectiveness of the preparation, and the cost of preparation. Both issues have been distorted by self-serving approaches.

The effectiveness issue usually overlooks one important issue, that of “potato chipping”. When a mixed technology panel, (signals on one side, and ground planes on the other) is run through an abrasive scrub (pumice or aluminum oxide), the panel tends to warp (potato chip effect) towards the ground plane side. This is because the laminate is stretched by the impact of the abrasive. Obviously this can be avoided by chemical cleaning.

However, chemical cleaning is usually thought to be very expensive, and for most of the people, using the more common cleaning technology, this is true. But this does not have to be!

The normal approach to cleaning in PCB fabrication is to microetch, microetch, and microetch. This approach is the reason why many PCB chemical vendors are making more money than their customers. The typical proprietary persulfate based microetch sold into PCB fabrication today costs 1¢/ft²/10 microinches of Copper removed. This means that the average fabricator using a microetch is paying 8¢/ft² of panel processed (40 microinches from each side), just to clean the innerlayer before photoresist. If your chemical vendor drives up in a better car than you are driving, now you know why.

The Other Approach

In response to the needs of the photo-chemical milling (PCM) industry, RD Chemical developed a technology for cleaning Copper foil that uses no microetch. This was done initially because the PCM industry does not want to roughen the surface of the foils during processing. It was later realized that this approach had a number of other important benefits, among them being:

1. Dramatically less expensive
2. More effective at promoting photoresist adhesion
3. Significantly less difficult to waste treat.

The approach starts with a powerful, properly formulated, alkaline cleaner for a prolonged cleaning, which removes everything, except heavy tarnish. This is followed by an acid (non-etch) cleaner, which removes any oxide (tarnish) remaining, and neutralizes any alkaline residues. And that’s all!

What is the catch? How can this work? The “catch” is that the alkaline cleaner step must be either significantly hotter, or significantly longer, than the PCB fabrication industry is used to using.

Specifically, the alkaline cleaning time/temperature combinations must be on the order of one minute wetted time at 150° F, or two minutes at 130° F, or some compromise between the two, in order to be able to out-perform a microetched surface. This does not mean that the method will not work at all if the cleaner is used for only one minute at 130° F, it just doesn’t work as well.

The acid clean step is mild. The cleaner is used unheated (70° F minimum) for 20-40 seconds.

Since the major consumption of both these cleaners is due to solution dragout, not actual “using up” of the ingredients, the cost of cleaning per square foot is on the order of 1/8, or less, of the typical cost of microetching, i.e.less than 1¢/ft².

This type of innerlayer cleaning approach has proven successful at some of the major PCB fabricators, (see May 1995 “Electronic Packaging and Production”) as well as some of the leading PCM companies. And if anyone needs resist adhesion, it is the PCM industry, who etches through 3-10 mil Copper (and Nickel-Iron alloys) using Cupric or Ferric Chloride solutions with aspect ratios that are often less than 1.00.

The key to getting this cleaning technology to work is clearly in coming up with an alkaline cleaner that is almost miraculous. And, as much as we would like to discuss freely the approach we took in developing such a product, there is always the possibility, however slight, that one of these newsletters could fall into the hands of RD Chemical’s honorable competitors, who just might then try to see if lab personnel could find the way to a similar approach. So, if you would like to hear about the details of this technology, I would be happy to discuss it with you personally.

To sum up, microetches are fast and convenient cleaners, but the convenience comes at a high price, there are inexpensive alternates, that work better.

Saving money in chemical processing, one square foot at a time.