Bill,
Your comments pose some interesting areas of concern...
I forgot to mention that this board only has one SMT component, more or less
in the middle of the board.
Years ago I had a larger board roughly 5 x 3.5 for which the assembler
recommended 0.015-0.020 final web thickness. That board was packed full of
SMT and THT components and had secondary ops as well, including two swage
standoffs.
Reasoning that this new board is of considerably less area and mass, with no
stressful secondary ops, I'd expect a thinner web thickness recommendation.
I made some phone calls and confirmed the 0.012 figure with several fab and
assembly folk. I've had confirming recommendations from this post as well.
One person at my company postulated that having 120 boards with all the deep
score lines might cause the panel to cup during pick and place. As I said
before, we want the boards to be broken out easily by hand, but the other
side of the coin is cupping or sagging, as you indicated. The assembler
thought it would be OK, though, for this first run.
I also forgot to mention the next assembly considerations. After SMT
soldering, these boards will be detached in groups of two columns each, to
be placed in a fixture for gang hand soldering of discrete wires. So I
mirrored the images are mirrored in each "even" column to keep the wiring
pads outboard.
I picked the 12.000 X 8.500 panel length because we needed to go in
multiples of two across. The basic array-only dimensions are 10.000 X
7.500. So I left an extra 1.000 east and west, and an extra 0.500 north and
south for tooling holes and fiducials at shop option. I, or shop, could
probably cut down the excess material to get a little smaller. Either way,
it fits within the 4-up guideline you mentioned, which is what I'd had in
mind to save cost, knowing that everything shears down from 36 X 48 to 18 X
24, 9 X 12, etc. for best material usage.
If the assembler has gas with the panel cupping or sagging we can reduce the
overall dimensions appropriately.
Nobody has come up with better IPC standards on this science, so I would
like to say thanks for your time and trouble!
Gary
-----Original Message-----
From: Brooks,Bill [mailto:[log in to unmask]]
Sent: Thursday, August 16, 2007 1:22 PM
To: [log in to unmask]
Subject: Re: [DC] V-scoring and web thickness
Gary,
I would recommend sending the panel design out to your board vendor and
having them make a blank .062 panel board with the V-scores in it for
testing the panel concept. Web thickness vs. groove depth is divided up
typically as 1/3, 1/3, 1/3 of the board thickness. But each design is
different, and not all shops do V-scoring exactly the same... some use a
diamond wheel cutter and some use routing tools... check with them before
adopting a strategy and see what works best for your assemblers.
V-scoring can be a real board weakening operation that can allow the board
to sag when put through a wave soldering machine, or Pick and Place holders,
or even hand stuffing operation with board supports... sometimes it can be
hard to predict just how a scoring pattern can affect the panel stiffness
and the ease of breaking out single boards from the panel...
Our vendor made a blank panel for us for free to test out a routing design
and make sure that the boards would not separate when they weren't supposed
to and that the combination of the number of boards and V-scores didn't make
the board too weak. This exercise can save you on board spins and headaches
in prototyping...
You may find that making a smaller panel is more to your liking when
handling during assembly and test based upon the size of the panel you are
proposing and the number of v-scores that will be required to have 120
boards in it...
Also, take into account the board manufacturers panel size and the need for
some waste area around the perimeter of the panel...
How many of your assembly panels will fit into an 18X24 inch panel?
We chose to make our assembly panels 8X11 so that they will not be throwing
away board material at the manufacturer that we pay for but would not get
boards out of... That size gets us 4 of our assembly panels in one of the
board manufacturer's panels.
Sometimes we make 1/2 panels ( 4when we do small boards like you are doing
to make them stiffer and I design supports into the panel to help with
rigidity as well.
Panel design takes some careful thought, and planning... Get to know your
assembly people well and ask questions... it will help your product be more
cost effective and less trouble to build. Good Luck with it... :)
Best regards,
Bill Brooks, CID+
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Gary
M. Koven
Sent: Thursday, August 16, 2007 12:05 PM
To: [log in to unmask]
Subject: [DC] V-scoring and web thickness
I'm preparing a panel layout drawing.
For purposes of this post, all dimensions are in inches.
Material is 0.062 FR-4
Panel size is 12.000 X 8.500
Board size is 1.000 x 0.625, 120 boards per panel
I want to v-score these boards for "easy manual singulation" and would
like to put a note on drawing to make sure the cuts are deep enough.
Now then,
READILY AVAILABLE REFERENCES
1) Apart from IPC-2222 Table 5 general parameters there appears to be no
other official IPC publications describing preferred web thicknesses.
2a) The best Internet reference I've been able to find is here:
http://www.tkb-4u.com/articles/other/vscore/vscore.php
2b) According to the matrix within this article, for a panel 10 to 15
inches long and individual PCB 1.0 long, I should recommend 0.010 web
thickness.
3) According to the LISTSERV archives, in message 006509 back in 2001,
my old colleague Ed Valentine (now retired) recommended 0.012 web
thickness for easy manual breakaway.
CONCLUSION
So if I didn't know any better and if nobody answers this, I'd end up
with a drawing note something like this:
"V-SCORE TO FINAL WEB THICKNESS OF 0.012" +/- 0.002" USING 30 DEGREE
CUTTERS"
I'm going to call some fab folk to get their take, but meanwhile, your
assistance here is kindly appreciated.
Thanks and Best Regards,
----------------------------
Gary M. Koven, C.I.D.
Dynazign, Inc.
Charlotte, NC, USA
704.405.1234
Veteran of the Marsh School
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