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]  

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************