Stouffer T2115 21 Step Wedge that is used for determining correct exposure times.
The Stouffer Step Wedge is used with Dry Film Photo resist, Dry Film Soldermask, contact positives, silk screen printing, printed circuit boards, silk screen stencils and most other photographic applications for judging results, determining speed and contrast for emulsions, and hardness of coatings.
The Stouffer step wedge also serves as a gauge to evaluate the variables encountered in the exposure and processing of dry film Photoresist, lithographic plates and photographic materials by contact. It indicates the direction and magnitude of exposure or processing adjustments necessary to achieve the desired results. Supplied with instruction sheet for how to use.
| Part Number | Steps | Density Increment | F-Stop Equiv. | Dmax | Size |
| T2115 | 21 | .15 | 1/2 | 3.05 | 1/2 x 5″ |
How to use a T2115 21 step to Determine Correct Developing Time for Dry Film Photoresist
Refer to the TDS Technical Datasheet for your Dry Film Photoresist or soldermask or photoresist. A Stouffer step tablet consists of a series of 21 steps which vary in density from totally clear (Step 1) to totally opaque (Step 21). For the UV wavelengths used in printed circuit applications, the ratio of effective exposure between any two consecutive steps is a constant factor of about 1.414 (i.e. square root of 2, or ½”f” stop).
To calibrate your UV light source:
- Laminate a piece of substrate with the Dry Film photoresist or Soldermask that you intend to use.
- Position the 21 step Stouffer gauge between the soldermask (or photoresist) and a clear area on your artwork. Be certain that nothing on the artwork overlaps the step tablet.
- Place into a UV exposure unit.
- Calculate an initial exposure time with the Fortex range of UV units this is normally around 20 seconds.
- Expose your “board” on the UV Exposure unit.
- Let the board rest for 15 – 30 min., strip off the Mylar cover sheet and develop as described below.
- When the pattern is completely developed, examine the image of the Stouffer gauge. The highest number step that still shows some material present after developing is your “exposure number“. The highest number that shows no removal of film is referred to as the “step held” or “Solid Step”. In the case of Fortex Dry Film Photoresist a Solid step of 6 or 7 is required, the “exposure number” should fall between 8 and 7. In the case of Fortex Dry Film Soldermask a clear metal step of 9-11 should be achieved with a Stouffer 21 Step Tablet wedge.
EXAMPLE:
If you get an initial exposure number of 6 (as shown, step held = 5), you should increase the Initial exposure time one full step to reach the recommended range for Fortex Dry Film; i.e. multiply by a factor of; Initial Exposure Time (seconds) * 1.414 =Exposure Time (Seconds). For Each additional step required multiply by another factor of 1.414 ie for two steps Initial Exposure Time * 1.414 * 1.414 = Exposure Time (Seconds)Then run the test again - If the initial exposure number is too high, your board is over exposed. Count back to the center of the desired range and divide your exposure time by the appropriate divisor.EXAMPLE:
If you get an initial number of 7, calculate the correct exposure time by dividing your initial time by:
Exposure = (initial time)/(1.414)
It may require as many as three test exposures to “zero” the process in but, once you have determined a working exposure time, it will always be a good starting point. This calibration should be conducted on every photoimageable material you will be using and the results recorded in your dated process log.
Instructions for Photo – Lithography
Set the contact printer / vacuum frame to make several trial exposures on separate pieces of film, with each exposure for a different length of time.
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 to repeat this testing procedure using different exposure times.
Process the test negative for the time and temperature recommended by the film manufacturer. It is important to keep processing consistent.
Examine the step tablet/wedge image on each of the negatives. Select as the best the negative in which step 7(on a 21 step wedge) 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/wedge. Use the step tablet/wedge as a benchmark for all exposures to achieve accuracy and repeatability.
To find the exposure factor to hold more or less on the 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
