10 Things to Consider When Buying Photoresist Stripper

The Technology of Photoresist Stripping

Stripping of most photoresist is, essentially, just an acid/base neutralization process. Yet photoresist stripping is usually unnecessarily costly, misunderstood, often a process bottleneck, and a source of many reject boards, and that’s on the good days. We will attempt to analyze why these things occur at the molecular level, and offer some cures.

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Stripping problems usually are the result of two performance deficiencies, either failure of the anti-tarnish system and/or wide variance of stripping speed and quality, with use of the stripper.

Most photoresist strippers start off giving adequate anti-tarnish performance when the stripper is fresh. At some time during the life of the stripper the panels start coming out tarnished. This is frequently thought to be the result of the anti-tarnish chemistry being consumed. This is not the case. The real reason that this occurs is that dissolved copper in the stripper is a catalyst which accelerates the tarnishing of the copper metal substrate. As the stripper is used, the dissolved copper content builds, and the stripper becomes increasingly corrosive to the exposed copper metal substrate.

Thus the tarnishing of panels in a used stripper is, in reality, a failure of the anti-tarnish chemistry to deal with the increased corrosivity of the copper contaminated stripper, and can be avoided by choosing a stripper with anti-tarnish chemistry robust enough to overcome the corrosivity of very high levels of dissolved copper. This choice can result in saving large amounts of money on stripper chemistry, as the stripper can then be used to its real capacity rather than being prematurely dumped because it is tarnishing the copper. This is the more obvious problem.

The dramatic decrease in speed of stripping with usage of the stripping chemistry, is another source of truly excessive stripping chemistry cost, and worse, of many rejects in the manufacturing process. It is usually the result of using general purpose resist strippers, and/or inappropriate control methods.

The general chemistry of most resist strippers used by the PCB manufacturers in the US is very similar, even though it comes in drums with different labels. Further many of the PCB manufactures are using the same stripping chemistry for stripping inner and outerlayers, and most of them using this type of chemistry are wasting 50% of the money that they are spending on resist stripping, and getting unpredictable stripping in the bargain. This is not a pretty picture.

Admittedly, these are some pretty brassy claims, but they are unfortunately too true, and I will proceed to show you why this occurs.

Photoresist is acidic, and the stripping process neutralizes thisacidity, and in the process of neutralization the alkalinity of the resist stripper is consumed. The neutralized fragments are either dissolved or dispersed into the stripper solution. What these neutralizing agents are, and how they work determines the quality and speed of stripping.

The two most popular resist strippers in the US have essentially the same stripping chemistry. This chemistry is a compromise chemistry that does a good job on outerlayers for the first 20%, or so of it’s life, and then is only really adequate for stripping innerlayers over the rest of it’s life.

Specifically the principal stripping ingredients in these two popular strippers are Choline and Monoethanolamine (MEA). Choline is one of a class of organics called “phase transfer catalysts”, and it can be described as an organic caustic. In the normal inorganic caustics, the active part of the molecule is the hydroxide [ ], while the Sodium or Potassium is merely along to neutralize the charge on the [ ]. Choline contains the same hydroxide [ ], but the inorganic Sodium or Potassium is replaced by an organic amine with a positive charge.

Figure 1

This “organic caustic” is called a phase transfer catalyst because it can carry (transfer) the caustic end of the molecule, the hydroxide [ ], from the water “phase” to an organic “phase”, like photoresist. In other words this little gem can actually facilitate the caustic dissolving in the resist itself. This is why Choline is the best thing that has ever happened to stripping photoresist. The Choline actually carries the caustic into the film, thus destroying the chemical bonds within the film, which breaks the film up quickly.

There is however one tiny problem, Choline is very, very expensive. It costs something like 10 times the cost of MEA per square foot of resist stripped. Again, Choline costs something like 10 times the cost of MEA per square foot of resist stripped. Monoethanolamine (MEA) reacts with the water in solution to produce the hydroxide [ ] by this reaction:

Figure 2
Now that we have covered the technical stuff, here is the critical fact that explains why strippers slow so dramatically so early in their life. Choline and MEA are very different strength bases, and because of this they are consumed sequentially (one after the other) in the stripping process, rather than together. In other words, all the Choline is completely consumed before any MEA is consumed. This results in a stripper of widely varying performance, as the stripper runs out of Choline, and shifts to stripping completely with MEA, the quality of strippiong decreases dramatically.

As we have all experienced, these widely varying performance characteristics can result in widely varying results, which are also known, in more polite circles, as rejects.

It is now time to get real real. The formula of the two largest selling resist strippers is approximately the following:

Figure 3

The interesting thing is that the 7% Choline costs twice as much as the 30% MEA, yet it strips only 1/4 the square feet of photoresist, however it strips it very well. For the purposes of this talk, let us equate speed of stripping with quality of stripping, and of course, in general this is not a bad approximation. This kind of copper technology results in a speed versus number of panels stripped curve that looks like this:

Figure 4

Notice that the speed over the first 20% of the stripper bath is very high, and almost completely uniform. Then as the Choline is depleted, the stripper starts stripping with the MEA, and consequently the speed drops suddenly and dramatically, and is uniform through it’s life. At this point the stripper is acting as if it were a pure MEA based product, and there was never any Choline present.

Luckily for the PCB fabricator, there is a gauge to tell exactly where a given bath is on this curve. This gauge is the pH of the bath. It is possible to replace the left axis of the speed versus panels graph above with a pH scale, and it will read just as true.

See this next graph:

Figure 5

Notice that the Choline containing stripper starts at a pH over 13, and when the pH drops to 11.5 there is no Choline remaining, so all remaining stripping occurs with only the MEA. This is a handy way to control this stripping chemistry, and a much better approach than the traditional titration, which does not address quality or strength of amine present, but rather only looks at overall quantity of alkalinity present.

This is further why controlling this type of chemistry by monitoring the pH, not by titration, is the best way to insure the quality of stripping. The way that this is setup is to run the line until the minimum acceptable quality of stripping is observed, and then check the pH. This pH becomes the minimum acceptable at which the bath gives acceptable results.

But there is a larger issue to consider. This is the issue of whether or not it is appropriate to be using a stripping chemistry of this type at all, and whether or not it is not better to have two separate stripping chemistries, one for innerlayers, and a second for outerlayers.

As expensive as Choline is, it is better than producing rejects by trying to strip outerlayers using only MEA. However, if Choline is the critical key to stripping the outerlayer, then the intelligent PCB fabricator would dispose of this type of stripping chemistry after the Choline was consumed. This would result in effectively throwing away 80% of the stripping capacity of the product.

On the other hand, when stripping innerlayers, most PCB fabricators have no need for the expensive Choline in the product, since innerlayers are, relatively, so easy to strip. All of this assumes that the stripping chamber speed is not being pushed to it’s absolute maximum, in which case innerlayers can require Choline, and only Choline, as the key stripping ingredient. This implies that for most innerlayer stripping, an all MEA product would work very well, avoiding the high cost per square foot of resist stripped by Choline. Since the Choline is 65% of the cost of the popular stripping chemistries, by putting all that money into MEA instead of a Choline/MEA blend, it is possible to more than double the resist stripping capacity of the stripper without increasing the cost per gallon. This is the performance curve of an all MEA stripper.

As you can see from this graph:

Figure 6

with an all MEA stripper, it is possible to save more than 50% of the stripping chemistry costs by using application-specific chemistry for each stripping installation. But let me make this even more clear with the next slide.

These are the chemical costs of the typical general purpose resist stripper.

Figure 7

Looking at this it is obvious that we are in the wrong business, we should be selling Choline. But it is probably too late for that. As you can see, the Choline, which represents only 20% of the capacity of the stripper, is 2/3rds of the cost! It is clear that it is important to use Choline only when it is really required.

For outerlayers, a very high Choline product, with little or no MEA would give outstanding, consistent results, and although it would be very costly per gallon, it would be very inexpensive per square foot of resist stripped.

Innerlayers can be economically and completely stripped using a Choline-free stripper that costs no more per gallon than the conventional mixed amine strippers, but that gives a nearly 100% increase in square feet of resist stripped per gallon.

There are many other issues that can be discussed about resist stripping, such as the effect of solvents, and Choline replenishers. It is also easy for the PCB manufacturer to determine what chemistry is really required, and then specify that chemistry to it’s vendor. But this would require a longer time, and getting more specific, than is appropriate to this general talk.

I hope this has been enlightening, and that I spoke slowly enough for my competitors to take accurate notes. It would be a shame for them to foul up the details when they are developing strippers based on these concepts. More important, I hope that the PCB fabricators can now better understand a chemistry that is key to their manufacturing process, and understand that they have some options that will allow them to increase their yields, and lower their costs, both in original chemistry costs, as well as in waste treatment costs.



Are you interested in learning more about Photoresist Stripper? Contact us today to secure an expert consultation!



As everything keeps getting smaller this has become one of the most important steps. A speck of dirt is enough to cause an open circuit on fine lines. The slightest misalignment on artwork top to bottom has the potential to shift dimensions outside the tolerances.

There are many different exposure units on the market now. Most do a very good job, but there are a few things that are important to have to make sure you can etch fine features.

Those features are collimated light (or LDI) and features that help create near perfect alignment. You will also need a yellow room. Keeping the yellow room and material clean will help you significantly.

The last tip we have is to hold the panel for 30 minutes after exposing it so the resist can lock into the metal more before developing it.

Approximate Cost = $200,000

The developer is used to dissolve the photoresist that was not exposed to the UV light in the above step. Removing the unpolymerized resist exposes the metal you would like to etch away. This provides you with the design you want to etch into your material.  

Developing is normally looked at as less critical than etching. This is true, BUT it doesn’t mean you can overlook the developing step. The photoresist can be underdeveloped or overdeveloped.

Underdeveloped means all the unpolymerized resist was not completely removed. So, you did not fully expose the metal that you want to etch away. This results in smaller holes or no breakthrough for chemical milling and shorts for circuits.

Overdeveloping will cause the developer solution to start undercutting the resist. This can cause your resist to lift while etching and reduce your etch quality. With resist lifting, you may not meet the specifications you need or you may receive open circuits.

Approximate Cost = $92,000

The etcher is where the metal removal happens and is the most critical piece of wet processing equipment. Like photoresists, the chemistry used in the etcher needs to be matched with the metal you want to etch.

What is so critical about the etcher?

The etch uniformity is first and foremost. To have great etch uniformity you need to have the solution moving. If the solution sits on one spot for too long the solution becomes less or non-reactive.

This causes poor etch uniformity. This is known as the puddle effect.

Secondly, transport is important. Most material thickness has no issue in the developer because all the metal is there, and the photoresist adds to the thickness. However, during etching, the metal is being removed.

That means the panel is becoming less rigid and prone to wrapping around a roller. (Most transportation issues only apply to thin material processing)

Buying an etcher that has great etch uniformity and transportation is key to building a successful etching shop.

Approximate Cost = $97,000

Stripping the photoresist from the metal is the next step. As a whole stripping is less technical than the other wet processes. Basically, the resist needs to be in the chemistry long enough to strip.

There are a couple of important factors to consider if you want an efficient stripping line.

First, you will need to check with your chemical and photoresist supplier to ensure that they are compatible with each other. If they are not compatible you will see your resist stripping off in sheets, as goo, as large pieces, or very fine pieces. All of which can create problems for your equipment and process.

You want your resist to strip in small chips. The reason you want small chips is that strippers are designed to filter out small chips. If you are getting any other size or goo, this means your resist and chemistry are a mismatch, and the stripper will have a hard time filtering out particles.

This will result in plugged nozzles.

Secondly, you will want to decide what design is best for your shop. Are you running high volume or just a couple of boards a day? This will help you determine how the filtering is set up.

You will also want to understand how the filtering works. If the machine you select does not filter the resist well, you will experience lots of downtime due to nozzle clogging.

No one wants down time, so make sure your stripper has an adequate filtering system.

Approximate Cost = $103,000

Many etching shops put all three of these processes in one line. This allows shops to use less operators because the panel will go through all three processes before it needs to be unloaded.

There are some negatives to putting all three together. The line is much larger, so you need a large room.

The processes do not run at the same speed so some chambers may need to be larger than a stand-alone machine. If you move forward with opening a shop a Chemcut employee can help you through this decision process.

Approximate Cost = $360,000  

There are multiple options for wastewater treatment, but the most popular is a batch wastewater treatment. A batch wastewater treatment system will consist of a collection tank, a treatment tank, filter press, clear well tank, ion exchange, and final tank.

The collection tank will collect all the runoff from your wet processing equipment. All your machines will have some liquid leaving the system. Mostly from your rinses.

The wastewater that is collected can be transferred to the treatment tank (precipitation tank). The wastewater will be treated here, and all the metals will precipitate to the bottom of the tank.

The clear water can be decanted into the clear well tank and if all the metals are removed the water can be transferred to the final tank. From the final tank, the water can be pH corrected and sent down the drain.

The metals (sludge) that settles at the bottom of the treatment tank will be pumped through the filter press. The filter press will remove all the sludge and this sludge can then be shipped off your facility as toxic waste.

The last piece of equipment you will need is a scrubber for the fumes that are being pulled out of your wet processing line.

Approximate Cost = $350,000

Contact us to discuss your requirements of Acetone. Our experienced sales team can help you identify the options that best suit your needs.