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Step-up stencil: recommendation thickness

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#82590

Step-up stencil: recommendation thickness | 20 May, 2019

I have a PCBA job which has on the top side a few standard QFN components (0.4mm pitch) and a big module (35mmx60mm) with 1.13mm BGA pads: https://imgur.com/a/YUGgejW

To be clear, this module is composed of a PCB with some components on top and some BGA balls on bottom side. This module lies on the top side of our PCB.

Sometimes, we have noticed that the FR-4 PCB of this module curves a little bit on one end or is not completely horizontal. It's like it's heavier on one side so it's at an angle. Very very tiny angle obviously but we have seen it on microscope. This could lead to some pads not connected to our PCB.

Therefore, we would like to use a step-up stencil at 0.12mm everywhere (for the QFN) and at a higher thickness for this BGA module.

This table is useful: https://www.7pcb.com/blog/stencil-thickness-calculations.php

I was thinking about 0.16mm for the step-up. What's the maximum thickness that is still safe for the 1.13mm pitch?

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#82591

Step-up stencil: recommendation thickness | 20 May, 2019

Resources are:

* IPC-7525A - Stencil Design Guidelines, 3.5.2 Step-Up Stencil This type stencil is useful when it is desirable to print thicker solder paste in a small portion of the stencil. An example would be a ceramic BGA where it is necessary to get 0.2 mm [7.9 mil] paste height because of ball coplanarity but 0.15 mm [5.9 mil] height on all other surface-mount component lands. In this case the stencil foil is stepped up from 0.15 mm [5.9 mil] to 0.2 mm [7.9 mil] in the area of the ceramic BGA. Another example is a through-hole edge connector that requires additional solder paste volume. In this case the stencil foil may be 0.15 mm [5.9 mil] thick everywhere except in the area of the edge connector where the stencil foil may be 0.3 mm [12 mil] thick. [NOTE: IPC-7525B: Stencil Design Guidelines [10/24/2011] is current]

* "Challenges for Step Stencils with Design Guidelines for Solder Paste Printing" Carmina Läntzsch, LaserJob GmbH, Fürstenfeldbruck, Germany, Georg Kleemann, LaserJob GmbH, Fürstenfeldbruck, Germany

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#82593

Step-up stencil: recommendation thickness | 21 May, 2019

Hello Greg,

Here are a few questions and advises that might post an answer:

1. How thick is your current stencil and what is the part that requires that thickness? 2. Can you use a carrier to keep your board straight and avoid board warpage? May be even use stiffeners? 3. If you have to print over on the large BGA go from 4 to 6mil or from 5 to 7mil stencil thickness in this area.

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#82597

Step-up stencil: recommendation thickness | 21 May, 2019

Thanks to @davef and @Evtimov for trying to help. It's useful but not exactly the answers I would like to get.

I'm going to reformulate my question. I'm writing in mm so that it's more precise:

I have a 0.12mm-thick stencil for some 0.4mm-pitch QFN.

There is a big BGA 1.13mm-pitch module that has a tendency to wrap or to be at angle which creates open pads.

I would like to have a step-up stencil at 0.15mm or 0.16mm below the module. My concern is the risk of shorts because of the added solder. At what thickness should I be worried for 1.13mm-pitch pads?

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#82613

Step-up stencil: recommendation thickness | 22 May, 2019

Greg,

this is a huge pitch. 0.16mm is safe for this size of part.

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#82620

Step-up stencil: recommendation thickness | 22 May, 2019

Thanks. It starts to make sense.

There is a formula at the bottom of this page: http://www.circuitinsight.com/programs/50456.html It says: Stencil Thickness = 2.64mil + 0.0831 * pitch of component in mil.

for 1.13mm, that gives exactly 0.16mm. So, 0.16mm seems to be fine.

To your mind and your experience, what would be the limit thickness possible: - the limit would risk to create shorts at 1.13mm pitch - the step-up would be too big from 0.12mm and it would start to cause other problems.

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#82631

Step-up stencil: recommendation thickness | 23 May, 2019

A little hard to tell without knowing your part details and behavior and your PCB design. I have seen part warping so much that will show opens joints on the sides and short in the middle at the same time. Theoretically you should have area ration of 0.66 when you design your stencil, which in your case should be 11-12mil stencil. However if the part is warping a lot or having a lot of metal volume in BGA balls, it might start shorting before the 10 mil paste height.

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