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Photo Imaging Exposure Calibration

Written by Chuck. Posted in Preparation

Photo image exposure calibration

Whenever you get a new supply of dry film photo resist or dry film solder mask you need to determine the correct exposure settings.

Read more below to see how this is done.

 

From the Stouffer web site, I started with the following guide, found at http://www.stouffer.net/using21step.htm.

Instructions

  1. Set your contact printer or vacuum frame to make several trial exposures on separate pieces of film, with each exposure for a different length of time.
  2. Make three exposures: one determined by past experience or the exposure normally used in daily production, one for less, and then one for more. It may be necessary repeat this testing procedure using different exposure times.
  3. Process the test negative for the time and temperature recommended by the film manufacturer. It is important to keep processing consistent.
  4. Examine the step tablet image on each of the negatives. Select as the best the negative in which step 7 (on a 21-step step tablet) is developed to a dense black, with step 8 only slightly dense. Step 7 will be the aim point for normal copy.

There are many different ways for exposing plates. There are an abundance of different exposure systems and processing techniques. The one constant is the step tablet. Use the step tablet as a benchmark for all exposures to achieve accuracy and repeatability.

To find the exposure factor to hold more or less steps on a Stouffer 21 step Platemaker Sensitivity guide follow the chart below.

To Change Exposure

1 Step

2 Steps

3 Steps

4 Steps

To Increase Exposure

1.41x

2.00x

2.82x

4.00x

To Decrease Exposure

.71

.50

.35

.25

The highest number step that still shows some material present is the "exposure number". The highest number that shows no removal of film is referred to as the "step held".

So, I laminated a piece of DuPont Riston Multimaster Series Resist onto a blank copper plated FR4 board. Using 25 seconds exposure as starting point, I imaged the Stouffer 21-step guide. I left this board rest for 15 minutes, then peeled the protective sheet and developed it normally.

StoufferTest1

The datasheet says to read the exposure value as the highest step. This means the highest step that still shows some material present. In my test, this would be step number 9 since this step has material present though it is about half as transparent as step 8.

According to the datasheet, this resist should be exposed at 7-9 on the Stouffer 21-step scale.

So I’m right in the range. If I wanted to lower my exposure to the middle number 8, I would increase my time by 1.41. 25 seconds times 1.41 equals 35.25 seconds.

If I wanted to move the exposure up two steps, I would multiply 25 seconds times 2.0 which equals 50 seconds.

If I wanted to move it down a step, I would multiply my initial exposure time by .71, with the result being 17.75 seconds.

For now, with this resist, I’m going to stick with 25 seconds