Fellow Technetters,

Looking forward to your comments on this

Solder mask Process
We use the conventional Flooding process for printing solder mask. ( Flat
bed and Double side coater)
Tack drying - Horizontal and vertical ( Ink used - Taiyo PSR 4000)
Exposing - UV exposing with silver halide film
Developing - Horizontal developing using Sodium Hydroxide.


The issue
# We have termed the problem as " Ring Mask on pad" or " Scum ring". See
picture below.

(Embedded image moved to file: pic02995.pcx)(Embedded image moved to file:
pic11942.pcx)


As seen from the above pictures, the solder mask in the knee region of the
holes does not develop.


Observations
The problem is seen in
# Boards of all panel sizes varying form 18"*24" to 22"*26"
# HASL finish boards only.
# Horizontal and vertical tack drying process
# Predominantly seen in Double side coating process ( Horizontal baking)

My question
1) Has this problem been noticed earlier?
2) What variation in the process could create this defect?
3) What controls have to be put in place to check the reoccurrence of the
defect?

cheers,
Madhu
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