From and OEM's perspective, I concur with Mark's answer below. It's
application dependant. It depends on what the critical thickness is for
next level of assembly.
Our default is to specify "over final metallization." However, we will
specify differently if depending on where the mechanical assembly (e.g.
card slots in a rack, connectors, LCD clips,...) is sensitive to thickness
of the pwb. That call out may include a detail on the fab drawing if it's
particularly critical but generally it's a note associated with the stackup
detail on our fab drawing.
There's a variety of terms that have been used depending on the
application. Off the top of my head, I've seen notes such as "at card
edge;" in the case of a clamshell housing that's degree of closure was
determined by the thickness of the pwb; or "over laminate at card edge" in
a case where the critical fit area was cleared of metal and soldermask; or
"over soldermask and copper." I'm sure there have been other more (or less)
precise terms used. You get the idea...
The answer is "it depends."
dw
At 05:15 PM 1/22/2003 -0500, Franck, George wrote:
>Mark,
>You need to ask your mechanical engineer what is the limiting feature in the
>design.
>
>If you have wrap around end launch connectors or a card guide, then it might
>be the laminate. If you have a case going over the assembly, which might
>interfere with components, then it might be the over the copper, excluding
>solder if you hand solder, including solder otherwise. If a gull wing
>surface mount connector sits on the Soldermask, then perhaps you need to
>control the thickness over the Solder mask.
>
>My quick review of 6012 did not find a 'default' definition of overall
>thickness.
>
>Over the years, I have often been surprised at what I got, when I didn't
>know what I asked for. I call them 'lessons learned'.
>
>Best Regards,
>
>George Franck
>"My opinions are just that."
>
>
>
>====================
>Hi All,
>
>Here's what was ordered:
>0.062" finished board thickness, SMOBC, HASL final finish, IPC-6012 class 2.
>
>Here's the question:
>Does finished board thickness include the HASL coating thickness?
>
>Thanks,
>Mark Hargreaves
>
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