Matthew,
You are right you do not need to have designators on assemblies,
a pick and place machine or AOI machine could care less. Now if you want
a tech to repair your assembly so you can increase your yields then I
would say that you need to have designators. It is very difficult to
have an assembly in a test fixture trying to hunt down the location of a
part looking at the assembly then looking at an assembly drawing but not
before you check the schematics. Have I lost you yet? Now do that in a
production environment with the customer riding you on why you are late.
The only time when this wouldn't matter is when a customer could live
with the excessive waste, and I haven't found one yet that is willing to
live with it.
When laying out an assembly you need to think of how your
decisions are going to affect the over all production of that design
then multiply that by the number of assemblies you wish to have in a
given year. All too often designers forget that what we design has to be
repeated and be repeated cost effectively. A board house will build you
what you want according to your drawings however, they will not build in
quality. I think that if you have a close relationship with your
assembly house you will find that yields will go up and on time
deliverables will increase. I am not saying you need to do everything
they tell you but it is worth it to listen to have that have to say.
As for the silk on the pads issue all my drawings state that
there shall be no silk screen on the pads. If the assembly house gets
boards in that have silk on the pads they can reject the entire lot if
that note doesn't exist then they are stuck with them. Silk on the pads
is not entirely a designer issue many designers in the states use
imperial and material may be purchased in Asia or Europe where rounding
errors can happen. Do not rely on someone else to cover you rear, bottom
line is a designer has to answer for what went wrong.
Regards,
Ted Tontis
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Matthew Lamkin
Sent: Wednesday, February 07, 2007 3:05 AM
To: [log in to unmask]
Subject: Re: [DC] Silk Screen Verification
I tend to agree with John here, I do not have silkscreen outlines or
names for 99% of my SMT components.
They are all dealt with by SMT assembly drawings.
IMO there is no reason for most of them to be there, why would a SMT
component need any outlines etc?
It's a machine that is placing them so it does not need to see where
they go, boards are usually too
dense to be able to put component names on for them all & if you cannot
get them all on then there is no point in putting just what can fit as a
drawing of the others will be needed anyway.
Only PTH components that are manually added or are changed during the
boards life need legend, board idents and connector etc do too.
Although legend on pads is really the PCB designers problem/fault, not
the board houses.
If the person that does the board leaves legend on pads & does not check
for it then it's their fault
and no one else's, and putting a disclaimer on a document is just a way
of weaselling out of doing it right in the first way.
You can easily just take the Gerber layer for the SMT side solder resist
and copy that to be a copper scratch layer though.
Matthew Lamkin
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]]On Behalf Of John
Parsons
Sent: Tuesday, February 06, 2007 11:07 PM
To: [log in to unmask]
Subject: Re: [DC] Silk Screen Verification
Foolproof! Not sure if there is such a thing;
- Eliminate the use of screen printed legend unless you really, really
need
it. Most boards are so dense these days that it is next to useless
anyway.
- Train your CAD librarian or whomever to design proper
footprints/outlines
to begin with.
- If it really is an ongoing issue with your fab shop, find another
- Legend on SMD pads is an IPC violation so if you are spec'ing a build
to
IPC requirements then, as with the note suggested below, you have a case
for
an RMA but this is the last course of action you would want to take as
it
costs everyone involved time and money.
Good luck,
John
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