First of all, OSP is NOT a new technology. If your vendor wants you to
switch to OSP, you WILL get a price break. If they don't want give you
a price break, DEMAND one! OSP is a simpler process than ENIG with
friendlier, and cheaper, chemicals.
OSP certainly has its disadvantages, but we use it almost exclusively.
We demand a one year shelf life, and when PCB's in the warehouse exceed
the one year shelf life, they pass across my desk for solder ability
testing before they can be used. They almost always have decent solder
ability (using no-clean lead-free solder), but if they don't have
reasonable wetting, they are sent back to the shop for a recoat.
However, you only get one shot at a recoat! The OSP process has a
"micro etch" step that eats away at your via barrel plating. Too many
times through the micro etch will leave the copper plating too thin in
the vias.
Concerning rework, if you print solder on the pads, you have a "Thick
HASL" coated board, so the number of allowable rework cycles is no
different than HASL.
OSP does have problems if you take a long time, meaning days, to
complete the assembly of your boards. This is especially the case when
using the wave solder on thick 0.090"- 0.120" boards. If you have a
continuous flow process, you won't have a problem. It also has the
problem in that it can be removed with many of the relatively benign
chemicals used to clean solder paste off the board. If you clean a
misprinted board, make sure that board gets built soon afterwards.
Be careful about your selection of OSP chemistries the shop is allowed
to use. Some will be too thin, some too thick, others discolor, etc.
OSP chemistries are not made equal, but the market is competitive and
the chemical suppliers are constantly making improvements; so chances
are, your vendor has already selected a good OSP chemistry.
The biggest disadvantage you will see with OSP is that the solder will
not always wet to the edges of the pads. Pretty much, where-ever the
solder paste is printed is where it will stay, compared to ENIG, where
the solder commonly wets to completely cover the pad and even the
sidewalls of the pad. DON'T be fooled into thinking that difference
means ENIG gives stronger solder joints. OSP or freshly cleaned copper
is the standard by which all other surface finished are compared.
Ryan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Harman
Sent: Friday, February 04, 2005 10:17 AM
To: [log in to unmask]
Subject: [TN] OSP process for PCB
Can anyone help me understand the difference between gold immersion and
the new technologies (OSP) ORGANIC SOLDERABILITY PRESERVATION, my
understanding is OSP, immersion silver and immersion tin are the leading
lead-free alternatives.
My vendor is asking us to change from a gold immersion to this OSP
process. What are the positives and or negatives for this process? What
do I need to look for? Are there any limitations that may affect RF
performance? Is this process better then gold immersion and if so why?
Comparing OSP to immersion silver or immersion tin, Is this the
direction of the industry? What about reliability?
Any assistance in this would be helpful.
Thanks
David
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