Hi Steve,
usually the Stencil is much larger than the PCV and is attached to some sort of
Frame in the Stencil Printer. Then the Alignment of Stencil and Copper features
is done optically using fiducial marks on both PCB and Stencil. The PCB
feducials need to be Copper features for devcent accuracy as each Layer of the
PCB is done in a separate Process and thus pisotopning tolerances and
geometrical tolerances apply.
PCB houses usually hand out Designrules that contain all tolerances they can
handle (Copper-to-Soldermask, Copper-to-PCB_edge, Copper-to-Plated Drill,
Copper to unplated drill...). Tighter tolerances most often result in higher
cost, thus you need to know the tolerances before you start your design.
Especially the Copper-to-Soldermask tolerance can hurt you bad if the openings
aren't matched to the tolerance.
Back to your question, if you have a stencil that has about the size of the
PBC, then you must be talking about tome Prototype PCB with Prototype Stencil.
These must be aligned manually. I prefer to use larger Stencil as it is easier
to handle, but for some reason Prototype PCB Houses like to use the same Size
for PCB and stencil. One way for you to align is to hand craft a Stencil
printer. Take two PCBs of the same thickness and scotchtape them to the table
in a right angle. Place your PCB into this "reference Corner" and align the
stencil. Then Scotchtape the Stencil to one of the reference PCBs.
If you component are very close to the edge, then there is not much room for
the scotchtape (that’s why I prefer larger stencils).
BR
Gert
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Bni I
Sent: Wednesday, March 30, 2016 9:41 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Class 2 PCB shrinking
Hi
My 12Layer PCB has been manufactured according IPC 6012 & IPC 6000 Class 2.
I measured my PCB and it is 100-200 um smaller than the stencil.
- BGA in the center og the PCB
- 100 um difference on the edges, in direction to to board outline.
And I can not solder my SMD's.
The manufacturer says, that this is accepted by the IPC-2222 Section 9.1.5.
or Table 9-1.
Which says
Up to 450mm (my board biggest dimension ~400mm)
Class 1: 0.35mm
Class 2: 0.25mm
Class 3: 0.15mm
How could the standard so tolerant to be?
How could one produce a product with 0.5 mm pitch BGA?
Should give more strict constraint, than the IPC standard? Is this a common
thing?
Thanks in advance
Steve
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