3-Minute Precleaning Technique

History:

Precleaning various metal substrates in order to bond dry film resists is always more effective when scrubbing or microetching to create a roughened surface. What this does, in effect, is increase the surface area,which increases film adhesion. This works well providing the change in surface appeareance is acceptable to customers. When it's not, then different cleaning methods must be used and the resulting adhesion is lessened due to a reduction in surface area. The trick is to be able to clean all undesirable elements from the panel including oxides, oils, fingerprints and the like, without redeposition; and for those metals that require it, passivate to achieve a smooth, oxide free surface conducive to good adhesion. Many of these techniques require multiple alkaline baths followed by acid pickles that can, in some cases, be a very lengthy process.

Many Degrees of Difficulty:

Of course some metals are harder to clean than others; some can be pasivated, and some not; some want to oxidize rapidly, causing process delays and poor resist adhesion. The question is, is there a better way? Is multiple alkaline baths and hours of long passivation steps the only way to get the job done? The answer is "No". There is a better way. Away of cleaning and passivating, in a single bath with the time measured in minutes.

A Different Method:

Precleaning for photo resist application to copper in from one to three minutes would hardly be a feat worthy of note in the world of circuit board manufacturing. But achieving an abbbreviated cycle of a minute or two in the precleaning of stainless steel or Kovar (Alloy 42) is another matter altogether. Yet there are companies doing it today, using an alternative process to conventional techniques. The end result is improved dry film adhesion, cleaner etched lines, and increased yields.

The Process:

The process involves a unique cleaning system that does not etch, and therefore doesn't change the surface characteristics of the metal. If the metal has a dull, matte finish, it stays that way. No change. If it has a shiny finish, it will stay that way, too, down to and including ultra-thin, mirror bright sputtered copper on Mylar. What does happen is that the cleaner removes contaminates that interfere with good resist adhesion, such as chromate coatings, fingerprints, oils, oxides and the like, and replaces them with a passivating film that protects the cleaned metal surface, and inhibits base metal attack even while in the cleaning solution itself. This same film serves to aid rinsing, retard oxidation, inhibit hydrogen embrittlement, and acts as an adhesion promotor for dry film.

Getting Used To It:

Cleaning with this technology takes some getting used to, as freshly cleaned parts do not behave in the usual and long accepted manner. Standard cleaning methodology calls for a water-wet part that will hold rinse water for an accepted standard time period when vertically drained, the time depending on the metal or alloy being cleaned.

Old Concepts No Longer Apply:

These concepts simply do not apply when using this new methodology. Instead, some metals, such as stainless steel, remain "wet" after rinsing, but remain that way much longer than can be expected when using conventional cleaning techniques; other metals will repel water like a freshly waxed car. This phenomenenon is disconcerting at first, but users quickly become accustomed to it. The question is always asked .." if the part repels water when it's clean, and we know it repels water when it's dirty, how can we tell the difference". In practice it is very easy. Once you have processed panels through the solution and rinsed them, the difference is readily apparent. The cleaned panels simply look clean and they repel water uniformly. You won't see uniformity on a contaminated substrate.

A Learning Experience:

Users are constantly learning new things about this cleaning method. At this point in time we know it is capable of processing 300 and 400 series stainless steel; mild steel; Kovar; brass; bronze; copper; beryllium copper; nickel; tin; solder; plated tin-lead; nickel silver; other cupro nickel alloys; titanium; heat oxidized gold alloys, and, of course, copper innerlayers and flex circuits. It is reasonable to assume that other applications will be found.

The Process:

Processing normally utilizes a standard volume make-up with a standard time and temperature. Certain alloys such as brass are better processed at room temperature in order to inhibit leaching of zinc. Spray operations are usually able to deal with this problem by simply limiting contact time.

Stainless steel accounts for the most usage at the present time, and therefore a little more is known about this alloy. Stainless is usually cleaned in a 10 - 15% solution of this product with no help from other chemistries. Immersion is generally one and a half to two minutes at 110° to 115° F followed by a water rinse. Because it's stainless, the surface will be water-wet. The panel is then dried and laminated with photo resist. In some cases wet lamination is employed. Total elapsed time is not more than 3 minutes. The end result is not just on par with conventional cleaning techniques, but an improvement over them. Photo resist adhesion is enhanced, lines are sharper, etched shoulders reduced, and yields increased. And, of course, the time it takes for the precleaning process is greatly reduced.

Alloy 42, also known as Kovar, is another metal that is being successfully processed with this technology. Information is limited at this time, but we do know it can be cleaned at the standard concentration and temprature in one minute or less, and it leaves a very hydrophobic (water repellant) surface that also serves to prevent oxidation during production delays.

Titanium is a tough metal that can be difficult to bond resist to, and difficult to etch. We know that thin sheets of this material (5 mils) have been successfully processed with this cleaner without scrubbing. We also know that resists used on thicker sheets (20 mils) do not hold up to extended etching without scrubbing. Beyond that we know only that this cleaner can improve results by improving overall adhesion.

Nickel Silver (60% Cu, 40% Ni) can be difficult to activate for good resist adhesion using conventional cleaning techniques. It can be, and has been, processed using this cleaning method with excellent results.

Aluminum can be cleaned and prepared for dry film lamination, but best results are achieved when parts are pumice scrubbed prior to immersion in the cleaning solution.

Copper Alloys such as brass, bronze, copper-nickel alloys, beryllium copper, etc., are easily processed with this cleaner. As indicated previously, leaching of zinc from brass is controlled by limiting contact time or reducing temprature.

Bonding Coverlays:

This cleaner, oxidation inhibitor, and adhesion promoter for dry films, has also been found to be an excellent bonding agent for polyimide coverlays on copper, adhering the adhesive to the copper with such tenacity that, once soldered, it will not peel. It has also been found to bond the so called adhesive-less polymides used for internal bonding of ridgidflex panels. Peel strength is no known, only that at least one company has passed mil spec. certification for rigidflex using this cleaning and bonding technology.

Nothing is Perfect:

Like anything, this new cleaner has its indiosyncasies. Rinsing is crucial to good results and some rinsing setups simply do not work well. For instance, in a tank operation using racks where multiple sheets have little seperation, difficulty in rinsing in an overflow rinse chamber with low water flow is a given. Recirculating rinse chambers in conveyorized spray machines are also bad news. By subjecting the surface film to a level of acidic bath residue, the passivating film is damaged and loses its water repellency in direct proportion to the strength of the build-up in the rinse water and the amount of time the panel is exposed to it. This in turn can affect the resulting adhesion for the resist and effect yields.

A Material of Many Uses:

This cleaning methodology has much to recommend it, and its rinsing limitations are minor and easily dealt with. In the cleaning of innerlayers and flex circuits it does four jobs in one: cleaning oils and oxides; removing chromates; inhibiting oxide formation; and acting as an adhesion promoter; all without a microtech. With flex circuits it is used a second time after resist strip as a cleaner before coverlay bonding. Onc circuit fabricator uses it after developing dry film on reverse treated foil in order to get clean etching in one pass, a critical part of fine line etching on thin copper.

The Future:

I mention the above applications only to point out that the full range of uses for this material has yet to be discovered. Certainly if there is a cleaning or bonding issue in your present or future plans, you should probably be looking at this new technology. It's a radically new way of doing things, but it works, and works well.