In answer to yokur questions about LPI solder masks, may I offer the
following:

1. Screen coating uses standard polyester fabric to screen apply the mask
onto the printed circuit board with a squeegee. Normal mesh counts range from
86 strands/inch (heavy caoting) to 137 strands/inch (light coating). The most
common is probably 110 strands/inch. Usually the screen mesh is totally open
(no stencil) and the mask is applied to the entrie surface of the PCB, with
only the edges defined by either "Blockout" or tape in a postage stamp
manner.

2. Curtain coating uses a continuous "curtain" of liquid LPI mask which is
applied to the PWB as is passes under the curtain at a high spped (usually
about 300 feet/minute). The amount of applied mask is controlled by the "slit
width" in the curtain head (usually 0.5 to 1.5 mm), the pump rate and the
conveyor speed of the PWB through the panel. The "curtain" of LPI mask falls
into a catch tray, is gravity fed back into the sump and a pump transfers the
mask back up into the curtain coat head (usually through a filter). The mask
recirculates continuously and small amounts of solvent are usually added to
maintain viscosity (sometimes automatically).

3. Convection ovens are the most common for drying LPI solder masks. they can
be either batch type or tunnel type. The heated air flows by convection over
the wet mask during drying where the solvents are evaporated out and
exhausted out of the oven. Temperature, time and air flow are critical
factors in correctly drying LPI masks. I am not familiar with "tiurbine" type
dryers being used for LPI masks. Usually, these are used to dry boards after
a wet process, such as scrubbing, developing, etc.

4. Time, temperature and air flow are the critical factors in drying LPI
masks. Extending the time (keeping the temperature the same) will tend to
more effectively dry mask in the holes (it is thicker and needs more drying);
however, too much time can result in overdrying and difficulty in developing
the mask from SMT pads, etc. The best bet would be to consult your LPI mask
supplier for advice on correct drying parameters.

5. Most LPI masks have polymers/thermal cross linking agents that will
activate above 275 deg F. Standard thermal cure parameters are 60 minutes at
300 deg F to ensure complete curing in a relatively short time. Going up to
325 deg F will probably not cause you any problems; however, it would not be
a good idea to go above that temp. You would risk heavy oxidation of the
copper, board warpage and possible embrittlyment of the mask.

I hope this information is helpful to you. If you have any additional
questions, please contact me (or Technet).

Regards,

Larry Fisher
Dexter Electronic Materials
[log in to unmask]

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]>   *
* with <subject: subscribe/unsubscribe> and no text in the body.          *
***************************************************************************
* 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]      *
***************************************************************************