Paul ,
This time I do not agree with with you :

>1.Plated tin-lead post-etch fused is still the cheapest finish. Reasons are
>no need to strip the etch resist, no need to pre-clean and pumice the copper
>before solder mask application, no HASL process. All that is needed is to
>solder condition, flux and IR reflow which believe me is much cheaper.
Most PCB manufacturers forgot how to keep the plated Tin-Lead solderable.
There is a lot of mess concerning distribution of composition of the plated
solder , thickness distribution . Than photoresist stripper and alkaline
etcher affect the tin-lead . For multilayers thermal treatment is required
and then all impurities are being fixed .
The most crucial point : for IR reflow is only one pass : in my life I never
was able to fix the boards , than once failed in IR reflow .
What about polyimide boards ?


>
>2.It is possible to fuse thinner deposits if you are not concerned with
>cosmetics. Very good results are achieved with thinner deposits on fine
>line.
>circuits because of the etch undercut which increases the amount of tin-lead
>over the copper trace.
>
>3.It should be possible to get close to 63/37 otherwise, as you say, the
>deposit may get into the pasty range and spoil the cosmetic appearance.
>
>The only drawback to fused tin-lead is that solder mask has to be applied
>over it and this can cause breakdown through wave soldering. More and more
>assembly is now IR fused solder paste so this finish could well be worth
>looking
>at again instead of going for even more expensive alternative
>finishes.
>
>Paul Gould
>Teknacron Circuits Ltd
>UK
Edward Szpruch
Eltek Ltd - Israel
Tel  972 3 9395050
Fax 972 3 9309581
E-mail :  [log in to unmask]

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################