With Flex, Sometimes You Gotta Break the Rules

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Sometimes in life, we need to break the rules. For example, in junior high I had a curfew but to have my first kiss, I had to break curfew. I got grounded, but it was worth it!

My last article was about reasons to follow IPC design and inspection rules. This time, we are discussing instances where, due to complex requirements, customers are not always able to follow the rules. I will also discuss some design options that will hopefully keep you from “getting grounded.”

Here are a few examples of when a customer would need to stretch the rules and supersede IPC-2223 and IPC-6013.

1. Insufficient real estate for IPC-suggested blind or buried via hole sizes.

2. Limited space requires bending a flexible circuit tighter than the IPC-2223 bend rules.

3. Insufficient real estate requires placing vias closer to the transition rigid-to-flex areas on a rigid-flex circuit.

4. Requiring tighter than normal tolerances.

Now, let’s go through these in greater detail.

1. Smaller than IPC-suggested blind or buried vias. As packages get smaller and real estate is at a premium, customers sometimes go smaller than the recommended ratio of 1:1 for blind or buried vias. Due to reliability concerns, the smallest recommended microvia is 100 µm (0.004”). If the design will not support 100 µm vias, it may be acceptable to drop to 75 µm if the 1:1 aspect ratio is strictly followed. So, the 75 µm via hole can only go 75 µm deep, or less. Be sure your flex supplier runs thermal shock testing on all plated vias and through-holes (especially blind vias under 100 µm) to ensure that they are reliable.

2. Bending a flexible circuit tighter than the IPC-2223 bend rules. A recent customer had a situation where a flex arm on a 2-layer flex circuit did not have room for a radius of 10:1 as recommended by IPC-2223. The circuit had to bend back 180 degrees upon itself. We recommended a forming fixture to create a repeatable bend. Within this fixture, a clamp was placed to ensure that once a droplet of super glue was added, the circuit bend was stabilized. This allowed a repeatable and reliable 2:1 bend ratio. It is critical on a reduced bend ratio application that once the circuit is formed, it is immediately constrained and not allowed to relax. Once all devices were in place and a few bends were made and glued, cross-sections were taken to ensure there was no cracking or elongation of the traces, or delamination in the coverlay. Since then, hundreds of thousands of circuits have been deployed in the field with zero quality issues. When the available room and the install scenario were considered, the solution we came up with was a success.

To read this entire article, which appeared in the March 2021 issue of Design007 Magazine, click here.



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