Photoresist trials and errors

I've been using Datak negative-acting photoresist chemicals (ER-71 photoresist and ER-8 developer) for decades. It seems a few years ago they made some change in the formulation. It's thinner than it used to be. I could do one or two fairly thick applications of it before and it would work well. Now a thick application tends to create pools of the photoresist on the board that dry in darker greenish or yellowish colored swirls that don't develop off the board very well.

I tried applying 3 or 4 very thin coats of the photoresist on my boards. This was better but I've noticed that it ends up kind of thin in places and after being in the etchant bath for a while, the etchant starts to penetrate the photoresist. This required taking the board out of the etchant, washing and drying it gently, trying to go over the weak areas with a Sharpie pen and then continuing the etch.

I noticed particularly last week that a board which was cleaned with a scouring pad like a Scotch-Brite left fairly coarse grooves in the board. The etchant would somehow penetrate into the grooves and leave scratch marks etched into the board. This was particularly critical on some 47 GHz boards I made, where some of the copper lines on the board are only a couple mils wide.

So I decided to use very fine sandpaper to clean the boards prior to coating with the photoresist. I used some 800 grit paper followed by some 1000 and 1500 grit paper.This makes an almost mirrorlike finish on the boards.

This seems to have greatly improved the quality of the boards. I'l let you know more after I've done some more experimentation.

Also, I've been using Coreldraw to create PC board patterns, making a negative of them and printing on clear transparency film. Coreldraw has a feature where it can create a negative of a vector image with the click of a button. I print them "upside-down" so the side of the film with the toner on it is in direct contact with the board for exposure. I found that my laser printer prints true black (RGB 0-0-0) as solid black on the film but anything less than black prints as a fine halftone pattern. I have to go over the whole black area with a Sharpie to darken it up in this case. Looking into why some of my patterns were coming out with this half-tone mesh, I found that for some reason my Coreldraw inversions were inverting true black to true white, but true white was being inverted to CMYK 0-0-0-100. This is NOT true black. I had to go and convert all the "black" in the negatives to RGB 0-0-0. They then printed solid black on the laser printer.

There's probably some setting in the program I haven't found yet that gets around this.  

Zack W9SZ