We were just talking about that very subject yesterday! (here at Cat)
This is the way I learned it back in the dark ages, but some of the
figures might have changed since then so take this with a grain of salt,
ok?
(OFFICIAL DISCLAIMER AT END OF TEXT)
The common traditional material size that board fabricators buy is 36x48.
Most vendors can't process that size material (they would need huge tanks,
for example) so they cut the raw material down to quarters, which results
in a more manageable 18x24 sheet size.
18x24 may be perfect for fab vendors, but some assembly equipment can't
handle anything that large.
So,
you have to design assembly pallets (that's what we call a board with
breakaway rails or an array of boards with rails) that are small enough to
fit through EVERY station of your assembly/test process.
For example, here is our maximums:
Screenprinter 24x20 max
Pick n' Place 11x17
Reflow 18x24
TH Insertion 17x17
Wave Solder 15x20
AND, Those machines also have MINIMUMS of 4x4, so our smaller designs need
to be in pallets, too!
So the goal is to figure out some kind of assembly pallet size that will
fit nicely on the fab material size, and also fit within the equipment
capabilities.
First of all, the vendor does NOT want you to use ALL of the material!
For a two-layer board, they will usually request that you leave a
half-inch all around the edge free, and for multi-layer boards, they like
one inch all around. This is so after they place your pallets in a nice
arrangement on the material, they still have room to add test coupons,
theiving, tooling holes, alignment features, whatever they want to do.
So if you start with the 18x24 fab size, the size of your pallets should
fit into a 16x22.
So you can see already that two 16x11 pallets would be about perfect,
because
1) you aren't wasting any material, and
2) That size just happens to fit the assembly equipment in most cases
(which is why they designed 'em that way, duh?)
BUT,
Maybe your real finished board size doesn't fit into 16x11 sizes very
well, so there is nothing preventing you from designing three 7x16
pallets, or ten 5x8 pallets, whatever. Just keep in mind that you need a
little extra room BETWEEN the pallets too, for routing and other fab
stuff.
ALSO, you may be in the habit of keeping components away from the board
edge so the conveyor has some board material to grab, but if you are using
pallets with breakaway rails, you can put parts right up to the board edge
and sometimes even overlap it (assuming its okay from a mech/mounting
point of view)
AND ONE MORE THING, if you are designing extemely complex boards with poor
yields, it might be a pain for your assemblers to have to identify the bad
ones on a pallet (X-outs) and reprogram the machines to avoid the
bad ones, so in some cases it is advantageous to just have one-up pallets
(only one design on each) with rails.
DISCLAIMER: The reason I said some of this could have changed is that (I
think I heard this right) they are fabricating whole 36x48 panels overseas
instead of chopping them down to the traditional 18x24 , and even have
material sizes that are slightly different dimensions than the 36x48 to
improve yeilds and lower costs. So you might not know exactly what they
decide to use, but if you do your job calculating the "traditional" way,
they will NOT have a problem.
best wishes,
Jack
Oh Yeah, you asked about Figure 5.1 in IPC-2221.
I think the intent of that diagram was to show BOARD SIZES (or I guess it
would be pallet sizes too) that would fit comfortably on a panel
The key point I got out of it is this; Say you were trying to make a board
fit into the 2.36x3.15 size, but you just couldn't do it. Since you
already lost a whole "row" of boards because they can't fit now, you might
as well go up to the next size, because you won't be wasting any more
material. Here's a better example, say you have a board that is 8x16.
well, you can't get two of them on a panel so since you are just wasting
that extra 3 inhes, you might as well use them to do a better design and
just make it 10x16 or 11x16 if your enclosure can handle the extra size.
That's what I THOUGHT they were trying to say, but the design I am working
on right now is 4.5x2.5 and I can get 12 per pallet. If I could scrunch
the board down to 3.8x2.2 I can get 16 per pallet, that is a HUGE savings
in the final assembly, because the bare board is the most expensive
component in the BOM and I just got 16 for the price of 12!
But if I can only scrunch it down to 4.3x2.2 I didn't save ANYTHING, I
might as well stick with the 4.5x2.5 and make my design job a little
easier.
Anyway, my numbers sure don't mesh with that 5.1 chart in 2221, do they? I
can't explain that....
"Brooks,Bill" <[log in to unmask]>
Sent by: DesignerCouncil <[log in to unmask]>
07/28/2005 05:47 PM
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"(Designers Council Forum)" <[log in to unmask]>; Please respond to
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Subject
[DC] Standard panel size?...
Caterpillar: Confidential Green Retain Until: 08/27/2005 Retention
Category: G90 - General Matters/Administration
We are re-evaluating our board assembly panel sizes and want to get the
best
yields from our vendor's manufacturing panels when we select the final
size
of our 'standard' assembly panel...
What is your PCB manufacturing 'standard' panel size for glass/epoxy type
materials??? Some PCB vendors tell me they have a working panel size of
18x24 inches... what do you use? And why?
Thanks in advance for your help...
Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
Datron World Communications, Inc.
_______________________________________
San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com
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