Hi Billy,
Hmmm... Do you mean that he wants to put uneven support under the board and
warp the board down to the uneven surfaces with the fasteners or screws,
leaving the assembly under compressed tension sort of like a catapult being
restrained from releasing its projectile? (not a great design practice)
I would tend to think under certain circumstances you might be able to
tolerate 'some' of that but in the extreme the laminate and the components
on the surface of the board would also be stressed the closer to the induced
warp that they were located near causing the high probability that they
would crack, fatigue, or damage the solder joint or material or degrade the
adhesion of the copper pad the part was soldered to the epoxy laminate...
not to mention the interlayer stresses between layers... but that really is
a judgment call by the designer and mechanical engineer...based upon their
knowledge of the design and the environment and stresses that the assembly
will be exposed to.
If they are concerned about it I would suggest that they consider reviewing
some options to even out the height of the mounting surfaces... (like using
shims or washers or something) to reduce the induced warp of the assembly
under tension and avoid stressing the board like that. Rigid board material
was designed to be flat...
Fiberglass laminate can take some tension but I don't remember getting any
data on how much... you may be forced to do some empirical studies on the
material you are using to determine the maximum tension the material can
take and then de-rate it for your particular application.
Another option might be a rigid flex design that could conform to the uneven
mounting points... :)
Best regards,
Bill Brooks
PCB Design Engineer, C.I.D.+
Tel: (760)597-1500 Fax: (760)597-1510
Datron World Communications, Inc.
Vista, California
-----Original Message-----
From: Billy Bharath [mailto:[log in to unmask]]
Sent: Wednesday, June 14, 2006 8:34 AM
To: '(Designers Council Forum)'; 'Brooks,Bill'
Subject: RE: [DC] PCB warpage tolerances
I may be wrong but I assume Brad wants to know the maximum warpage a pcb can
accommodate in its assembled form.
The word standoff is used which indicates the pcb could experience stress
during installation hence the assembly departments concern.
There might be standards covering this, I have not seen any, however I use
the rule of experience, on multi layer pcbs I use the IPC-A-600. On double
or single sided pcb I use past experience and examples
thanks
Billy Bharath
Cad Technologist
Prism Holdings
Kwa-Zulu Natal
South Africa
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]]On Behalf Of
Brooks,Bill
Sent: 14 June 2006 02:34
To: [log in to unmask]
Subject: Re: [DC] PCB warpage tolerances
Brad,
Here's a note that I use on my fabrication drawings...
"BOW AND TWIST SHALL NOT EXCEED 0.75% (.0075 IN PER IN) IN ACCORDANCE WITH
IPC-A-600 FOR SURFACE MOUNTED BOARDS WHEN MEASURED USING IPC-TM-650, METHOD
2.4.22."
If you are doing thru hole technology you can relax that requirement to 1.5%
(.0150 IN PER IN)
Most surface mount assembly vendors like to have a very flat surface upon
which to place components... the uniformity of the thickness and shape of
the solder paste printed on the lands is affected by how well the paste
screen mates with the surface of the board too. Obviously the paste will
bleed out under the screen opening edges loosing its shape definition if the
screen is not right on the surface of the board. Also controlling the
uniform height above the surface of the board affects the difficulty in
programming the pick and place machines and handling the parts in pick and
place... they need to locate the part in the X,Y and Z planes and if the
board isn't flat they can be pushing the part too hard onto the surface or
not close enough... Sometimes they can clamp the board in tooling fixtures
or heat the boards under pressure to flatten them and compensate somewhat
for poor bow and twist if it isn't too severe but the best way to control it
is through good design practice. Use of evenly distributed copper areas and
symmetrical layer stack up in your design helps a lot.
I hope that helps a little...
Best regards,
Bill Brooks
PCB Design Engineer, C.I.D.+
www.pcbwizards.com
-----Original Message-----
From: Stephen Chavez [mailto:[log in to unmask]]
Sent: Tuesday, June 13, 2006 4:42 PM
To: [log in to unmask]
Subject: Re: [DC] PCB warpage tolerances
Brad,
I think it's the following:
"Warp or twist of board shall not exceed .010 inch per inch"
I'd like to see other responses on this issue.
Regards,
Stephen V. Chavez
Vista Systems
602-943-5700
www.vistasystems.net
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Jaurique, Brad
Sent: Tuesday, June 13, 2006 4:35 PM
To: [log in to unmask]
Subject: [DC] PCB warpage tolerances
All,
We have a design that sits in a mechanical
housing that has crevices and milled out areas. Due
to this the standoffs are different heights and sizes.
There are of course mechanical tolerances with these.
My question is what is the IPC standard for pcb warpage tolerances. My
assembly department is telling me that we don't meet these standards yet
can't define what they are.
All help is appreciated.
Brad Jaurique C.I.D.
Senior PCB Designer
Pelco
Office - (559)292-1981 ext. 2347
Cell - (559)940-3966
[log in to unmask]
Tell me and I forget; Show me and I remember; Involve me and I understand.
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