Dry Film Photoresist Processing Technology

Citation

Ellis, B. (2002), "Dry Film Photoresist Processing Technology", Circuit World, Vol. 28 No. 1. https://doi.org/10.1108/cw.2002.21728aae.001

Dry Film Photoresist Processing Technology

Dry Film Photoresist Processing Technology

Karl DietzElectrochemical Publications Limited, IOM432 + xv pp.; 22 tables; 270 figures;338 referencesISBN 0 901150 39 8£85 (US$170)http://www.elchempub.com/epfiles/ep152.htm Keywords: Photoresists, Film

My biggest shock when I opened the packet containing this book was the dust-jacket: although in the traditional EPL style, it is coloured in the sexiest pink you could imagine, something to do with stripping, perhaps! My second shock was that a book this size could be written about such a specialised subject. A quick glance through will, however, reveal that the author knowledgeably touches on many themes which, although relevant to the main topic, are only incidentally so. This is not a negative criticism, rather the contrary, because it may avoid the reader having to check through more than one book to find an overall solution to a problem he may encounter.

A list of the 18 chapters will reveal the logical sequence and thought processes of the author:

1. 1.

   "Photolithography in printed wiring board fabrication".

2. 2.

   "Comparison of circuitising [sic!] processes".

3. 3.

   "Conductor via (through-hole) formation".

4. 4.

   "Initial through-hole metallisation".

5. 5.

   "Copper surfaces and base material construction".

6. 6.

   "Prelamination surface preparation".

7. 7.

   "Lamination".

8. 8.

   "Tenting".

9. 9.

   "Phototools".

10. 10.

    "Exposure".

11. 11.

    "Development".

12. 12.

    "Electroplating".

13. 13.

    "Etching".

14. 14.

    "Stripping of aqueous resist".

15. 15.

    "Safety and health".

16. 16.

    "Waste treatment and disposal".

17. 17.

    "Water quality in PWB fabrication".

18. 18.

    "Recycling PWB scrap".

Let it be said from the start – if you are seeking information on anything other than the various aspects of negative working, aqueous processed, dry film resists, you won't find much in this work. On the other hand, if you are wanting information on this type, then this book is for you. The author works for one of the major manufacturers of such materials and this does come through, but not too obtrusively. Another minor point of which the potential reader should be aware is that dry film solder masks are hardly considered in this book, other than en passant in Chapter 1. As the author honestly explains, this is because liquid photoimageable (LPI) resists have largely supplanted the dry film ones for this application, whereas the opposite is true for imaging the conductor patterns.

In early 1969, I was instrumental in starting a new PCB production facility and I believe we were one of the first, at least in Europe, to opt for 100 per cent production using dry film conductor imaging, with both direct etching and pattern plating. I recall that we had hell's own job with the early dry film resists and it took us several months of trials before we could obtain acceptable results and yields. However, perseverance paid. A year later, we were successfully using such resists to etch experimentally conductor widths and spacings of 40µm each on 35µm thick copper. This performance is still state-of-the-art. In a way, this book took me back through these difficult times. Of course, the chemistry and equipment have changed since then, as well as the knowledge of the manufacturers, but the basics remain, over 30 years later.

With this introduction to the technology in mind, I must personally disagree with the author on one point on which he weighs heavily in Chapter 6. This is the value of the water break test on a copper surface, which he appears to consider highly as an indication of the quality of the surface preparation. I found that the results from it, in terms of resist adhesion, were very variable, indeed. This is because of three phenomena: a rough surface does not have the same water-break characteristics as an otherwise chemically identical smooth surface; hydrophilic surface contamination, which can cause adhesion problems, actually promotes good results in a water-break test; and the degree of oxidation (even within seconds of cleaning) of the copper can change the results dramatically. Along similar lines, Dr Dietz has unfortunately omitted to correlate solderability with the various methods of preparation of the copper surface, something which is important if no barrier plating is subsequently done over the prepared surfaces.

I particularly liked Chapter 7, which goes into fine detail on the problems that can befall the most critical process of lamination. This is really excellent. Like many of the other chapters, it ends up with a good tabular summary of the critical parameters and features. I wish I had thought of doing something similar in my books! The chapter on electroplating is also very good, especially where it deals with the problems inherent between the resist and the plated metals, such as overplating and resist breakdown. It even goes into some details of the nature of the various types of plating bath commonly encountered. The chapter on etching is treated in an analogical manner, in equal detail.

PCB assemblers are sometimes bugged by the effects of small amounts of poorly developed or poorly stripped resist. The chapter on stripping is excellent at pointing out the possible reasons, even if faulty stripping reflects on the quality control of the fab shop – valuable help when discussing problems, indeed. This chapter also makes fascinating reading and I learned quite a lot from it. I was quite surprised, though, that there was not more information about the effect of temperature, one small graph and one laconic phrase stating that higher temperatures result in higher strip speed plus an equally laconic mention in the tabulated summary at the end of the chapter, stating that the temperature should be checked with a hand-held thermometer once per week! No correlation between the effect of the stripper and, say, polyimide substrates is discussed.

I was a little surprised that the chapter on safety and health (or "safety of health", as the page headings would have it, not the only proof-reading error!) is really concerned only with the resist itself. When one considers that the stripper is often a solution of sodium hydroxide and monoethanolamine and other additives, my guess is that a splash of the solution in the eye of an operator could cause severe problems, possibly even blindness, if first aid cannot be immediately administered, not to mention that monoethanolamine is considered very toxic (2-5ppm operator exposure time-weighted threshold limit value, according to the regulations in force). Yet, stripper solutions are not even invoked and developer ones only because of the dissolved resist.

The chapter on waste treatment and disposal is good, provided that one can translate the US into local regulations. There is a "decision tree", which is a good first guide as to what one should do with spent developer and stripper solutions. What I did not realise is that some of the proprietary strippers are so loaded with complexing agents that the metal concentrations can reach hundreds of ppm. This must be of great concern to many manufacturers who might otherwise be in ignorance and, in some countries, these strippers either would need critical waste water treatment or would be treated as hazardous waste.

The penultimate chapter, on incoming water quality, is interesting, because it treats the problems differently for the various processes. The last one is very short, fewer than two pages, including a process flow diagram of over half-a-page. It outlines a plant in Germany treating bare-board scrap, with a reported zero waste in the output. This is interesting, but I wonder about the overall economics of the operation.

The book itself reaches the usual high standards of the publishing house, both in physical presentation and in contents. The author has obviously tried to "go metric", although he is very erratic in his attempts, gleefully mixing imperial and metric units within the space of a few lines in some places – rather surprising, since he was born and educated in Germany. In others, both are given. Despite these Brian Ellis-like niggles, it is a good book, well written, easily readable and, above all, informative.

It is completed with a single list of references (not the usual chapter references, as in most of its sister-books) and what I feel is a slightly meagre index. I would say that it is required reading in every PCB fab shop that uses, or contemplates using, dry film resists, maybe even those that don't! It should also be in every assembly shop where:

• •

  dry film solder mask is on the boards;

• •

  high-tech assemblies are made;

• •

  the PCB fab supplier quality leaves something to be desired;

• •

  concurrent engineering is practised;

• •

  the potential reader is simply curious about the processes to which his bare boards have been subjected.

Now, let's see, just whom does that leave out?

Brian EllisCyprus