Ahhhh...what's good for the bare board fab house isn't necessarily good for the assembly house.
OSP IS a wonderful finish -- it's environmentally friendly and provides for a flat surface. BUT...gloves should be worn during assembly to prevent staining during handling, and, if working with a double sided board, the solderability of the second side may be affected as the OSP on that side degrades as the board is processed the first time through a thermal process (i.e.: SMT reflow). If working in a facility with a continuous flow line with minimal handling it works well, but, working in a smaller facility with prototype lot runs, OSP can be a nightmare especially if components are supplied by the customer on small strips and must be hand placed!
TIN. Now Tin's another story. I read that Jason Gregory loves the stuff, but, in the environments I've worked in we have a two sayings.... "The Sin of Tin!" and "Nothing solders better than solder but HASL is a Hassle!"
From an assembly standpoint, Tin is great for single sided boards with very fine pitch comps, BGA's and flip chips. It also provides for a non-lead plating surface solution as the industry moves toward (or hopes to move toward) lead free. The problems I've seen with tin include the following:
1. Insufficient Plating thickness or inconsistent plating thickness from the supplier.
2. Poor wettability, especially if using a low activity no-clean!
3. Oxidation due to the growth of copper-tin intermetallics making for the worst wettability I've ever seen. This occurs when first reflowed.
4. Tin Depletion while in storage.
Like the OSP, if the application fits for Tin, then use it, but, in many applications, neither finish is optimal. In many cases, Gold works better as it provides for a nice flat surface, but, again, if using a low activity no-clean, solderability will be poor. In sum, I've found OSP and Tin to be difficult to use in applications where both sides of the boards are populated with SMT devices.
-Carrie
-----Original Message-----
From: Dougal Stewart [mailto:[log in to unmask]]
Sent: Thursday, August 29, 2002 4:59 AM
To: [log in to unmask]
Subject: Re: [TN] Talk to me about Omikron White Tin board finish,
please.
I still don't understand why you assembly people don't use OSP ! It is the
easiest process to use in bare board fab, it is reworkable at the fab house,
if you have kept the parts on a shelf for too long and are suspicious that
they may not solder so well, you can send them back to the FAB for
reprocessing (assuming they have not gone bust meantime, or moved half way
around the planet) and you are not contributing to polluting the planet with
Thiourea, or killing the board shop operators ! So what has white tin over
OSP?
Dougal Stewart
email: [log in to unmask]
phone: +44 1896 822204
mob: +44 7984 629667
----- Original Message -----
From: "Jason W. Gregory" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, August 28, 2002 11:03 PM
Subject: Re: [TN] Talk to me about Omikron White Tin board finish, please.
> Barbara,
>
> I love white tin! It is absolutely, positively my favorite finsih. The
flatness,
> which is it's main attribute, is perfect for fine pitch, 50mil and lower,
and
> the wetting characteristics are superb. The only downside is the thiurea
(sp?).
> Apparently it's pretty toxic and carcinogenic. I'm sure Brian Ellis will
chime
> in soon (I hope).
>
> Jason Gregory
> Production Supervisor
> LaBarge Inc.
> (918)459-2367
> [log in to unmask]
>
>
>
>
> Barbara Burcham <[log in to unmask]> on 08/28/2002 01:29:06 PM
>
> Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please
respond to
> Barbara Burcham <[log in to unmask]>
>
> To: [log in to unmask]
> cc: (bcc: Jason Gregory/LABARGE)
>
> Subject: [TN] Talk to me about Omikron White Tin board finish, please.
>
>
>
> Who is using this and is it working for you?
> What do y'all think about the "Omikron" process as opposed to other White
Tin
> applications?
> Is one better than the other?
>
> Barbara J. Burcham, C.I.D.
> Sr. PCB Designer
> Fairfield Industries
> Sugar Land, TX 77478
> 281-275-7687 Work
> 281-275-7550 Fax
> [log in to unmask]
>
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