Tom,
Here is my two cents (pence).
As a finish Gold ENIG is almost always superior. Recent discussions at
the PCB Design Conference see almost all of the big guys going directly to
ENIG. You are right, inorganic or immersion tin is typically cheaper but it
shouldn't be a huge or significant price differential unless you are talking
high volumes. I would guess that you are not in the medical world.
My personal experience with Inorganic Tin, haven't seen a tin whisker.
I used it for many years at a former employer. My thoughts, could the whisker
problem be relevant from one process supplier to another? For your reference we
used Omikron Plus from Florida Cirtech Inc..
http://www.floridacirtech.com/login.asp If you want to find a fabricator
using it in your area.
Other's comments about FR4 need some further qualification. This is a
bit of a black art at the moment but close work with your fabricator and the
laminate supplier should see you through it. Don't fall into the trap of think
that simply specifying a higher Tg material will get you through. It is
reported that some High Tg (175deg. or greater) Fr4 materials actually do
poorer during Lead-free processing than some lower Tg (140deg.) materials. The
IPC is working on new tests and definitions for laminates used for Lead-free
processing. However they have not been finalized yet. I believe these will be
ratings will be called T266, T288 and Td. They will be derived from tests at
266 celsius, 288 celsius and I believe it was 300 or 305 celsius. The Td is
time to destruction (de-lamination) at the specified temperature.
Somebody commented that the additional temperature for Lead-free is not
that great, wrong! The processing typically hits temps 40 celsius higher
(approx. 265 - 270 celsius) than leaded processing and the boards are held at
that temperature for much longer periods of time, several minutes verses less
than a minute. As well the ramp up and down are much slower. So the boards are
at an elevated temperature for much longer periods of time.
Good luck, most of us are just behind you somewhere. My employer is
waiting and praying for the avionics exemption to go through.
Sincerely,
Brad Velander
Senior PCB Designer
Northern Airborne Technology
1925 Kirschner Rd.,
Kelowna, BC, V1Y 4N7.
tel (250) 763-2329 ext. 225
fax (250) 762-3374
-----Original Message-----
From: Tom Robinson [mailto:[EMAIL PROTECTED]
Sent: June 27, 2005 12:15 PM
To: [EMAIL PROTECTED]; Protel EDA Discussion List
Subject: [PEDA] Finish: Matte tin vs ENIG ?
With LeadFree coming, I'm trying to find out what is the best all around
finish requirement to put on my fab drawing.
My boards have a 50mil pitch BGA. The soic's down to .4mm pitch.
Matte tin, ENIG ?? I imagine Matte tin would be the cheapest.
Also, with the extra heat needed in reflow, will FR4 be ok?
Any help will be appreciated.
tr :)
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