HASL just isn't a flat finish!!!!
If you want HASL you must accept this. I prefer that there is a healthy dome
with as much shiny solder as possible. I have in the past had problems where
intermetalic growth has come through to the surface of HASL pads after
multiple thermal cycles, due to the vendor trying to achieve flat HASL pads.
Lets face it the only way to get a flat finish is to reduce the quantity.
You need to have free tin on top of the tin/copper intermetalic to maintain
solderability. If you want a flat finish, use one of the alternative
finishes. HASL is not a flat finish. I have just phoned our supplier to
complain about the flat finish given to us recently on our PCBs (PWBs). When
the quantity of solder goes down you start to get patches of dull solder
appearing which indicate a lack of free tin on the surface. I would rather
achieve a minimum 2 um on edges or the knee etc..( I know it's more usually
1 um), to maintain solderability, and see lots of shiny stuff!!!

Why would you want a flat finish? Yes there is improved print performance!
Yes there might be issues of coplanarity for non-eutectic BGA spheres and
long fine pitch SM connectors! But if you have adequate pad design, these
are non issues. We use HASL through out, for assemblies including large BGAs
with non-eutectic spheres, extensive use of 20 thou (mil....why can't we all
use the same technical phrases) pitch on SM connectors, large QFPs,
TSOPs,TSSOPs etc....

Soooo!!! Unless your dealing with wholly inappropriate pad geometries then
why oh why would you want a flat HASL finish?

At 14:06 19/03/98 -0700, you wrote:
>Dear Technetters:
>
>Today's question relates to HASL surface flatness. I have searched the
>Technet Archives and found no message where anyone would say definitively
>what the numbers should be, IPC-6012 is no help "Coverage and Solderable"
>with today's need for flatness when using SM Technology. Is there a
>specification that calls out a real number for the flatness of the HASL
>process. My questions come from a request by one of our PCB vendors to
>supply him with such inf. our Assy. personnel have no idea what it should
>be but know that what we get at times is suspect.
>
>I seem to remember talking to James Blankenhorn @ Mexitronica last year on
>this subject and wouldn't you know it I can't find my notes from the
>conversation. So Jim if your out there mind taking a shot at this.
>
>Steve Collins
>PCB Design Supervisor
>
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