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January 1997

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Subject:
From:
"Bert Ohlig" <[log in to unmask]>
Date:
Thu, 16 Jan 97 15:00:26 UT
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Just a small addition to the list below.

It is rare that the spectral sensitivity of the radiometer photocell matches 
perfectly with the spectral sensitivity of the LPI. The numbers provided by 
the manufacturers are biased on the photocell they used in their lab and 
usually based on a mercury lamp spectrum. 

In recent years metal halide additive lamps have become the predominant lamp 
in the industry due to their efficiency, intensity, versatility, and lamp 
life. Now there are a variety of spectrums available.

The spectrum of the lamp affects the:
	* exposure speed 
	* resolution 
	* surface tackiness
	* depth of cure

Control of the production involves a combination of, millijoule readings for 
short term machine confirmation and calibration, gray scales to ensure cure, 
and resolution targets to ensure consistent performance in the event that a 
lamp would change spectrum toward the end of it's life. 

	Bert Ohlig
	OLEC Corp.
	[log in to unmask]

----------
From: 	[log in to unmask]
Sent: 	Wednesday, January 15, 1997 12:15 PM
To: 	[log in to unmask]
Subject: 	Re: Photo-initiation

There are many, many types of photoinitiators used in the liquid,
photoimageable (LPI) solder masks on the market. Some are faster and some are
slower. Some respond to 365 nanometers, others at higher wave lengths, others
at lower wave lengths. On and on.

Putting a millijoule requirement (for example, 300 mj) for LPI solder masks
can be very dangerous because there are so many factors that effect the
correct exposure, such as:
 
1. The thickness of the solder mask deposit. Thicker deposits require more
energy than thinner ones.

2. The wattage of the exposure unit. Higher wattage units (> 7 KW) will
expose at lower millijoules than lower wattage ones (<3 KW).

3. The amount of heat buildup during exposure. Higher heat buildup will
expose faster than low heat buildup. However, a high heat buildup can cause
problems of artwork growth, sticking of the phototool, etc.

4. The d-min of the phototool used. Lower d-min will expose faster than
higher d-min.

>From my perspective, it is much better to use some type of step wedge (the
Stouffer 21 step sensitivity guide is the most common one here in the USA) to
determine the correct amount of exposure rather than an absolute energy
level. Most of the LPI masks that I am familiar with provide the best all
around performance (good sidewalls, good resolution, etc) with step wedge
readings of clear metal 10-12.

Good luck and don't hesitate to contact me if I can be of further assistance

Larry Fisher
Dexter Electronic Materials
[log in to unmask]  

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