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 A World of Thanks
 By Dale Baird

 Flex Material
 By Dale Baird

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 Presidents Letter
 Words From Mark Lencioni

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 Design Guide
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 Information on Flex Materials

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Welcome to the second edition of our internet newsletter, The Flex Files. It has been awhile since we've been able to concentrate on anything other than surviving the printed circuit board recession of the past two plus years.

Hopefully you enjoyed our first issue in January and will find something of interest to you about flexible circuit technology in each of our following issues. If you have any requests, please send them in and we'll either reply directly back to you or create an article that addresses your topic as it might be helpful to our other readers as well. Your feedback, good or bad, is encouraged as we have a goal of continually improving the content of this service to you.

We make an assumption in our articles that you are familiar with rigid printed circuit boards. As such, our articles will primarily address the engineering, design, manufacturing, assembly and quality differences inherent in flex and flex-rigid circuit technology.

Our second issue addresses flex drawing call-outs defining constructions and materials. We picked this topic to help improve your documentation before it goes out for fabrication. Lenthor Engineering receives a fairly large percentage of drawings that have to be clarified and discussed prior to releasing them to our shop floor for manufacturing. Working together, we can save time to delivery and reduce the associated costs.

I also wanted to express a world of thanks to our many customers that contributed to our being in business today. We appreciate your continued support and look forward to being a valuable part of your growth and success in the years ahead.

Dale Baird
V. P. Sales and Marketing
Lenthor Engineering LLC

How Thick is 1 Mil Flex Material?
Drawing Call-Outs for Flex Circuits

When it comes time to document your drawing with material notes (call-outs) or construction stack-ups, it's important to know the make-up of available flex laminate, coverlay, bond-ply, sheet adhesive and stiffener material.

A call out for 1 mil base laminate with 1 ounce copper both sides is not enough information for us to use. In rigid core laminates, the dielectric thickness is expressed in inches and the copper weight is added from there. In flex, only the polyimide film thickness is referred to along with the copper weights (in ounces per square foot). What is missing from all adhesive type flex laminates, the most common type, is the adhesive thickness used to glue the polyimide film to the copper sheets. This can be confusing for the engineers and designers new to flex circuit technology. A call out to use 1 mil base polyimide flex material, unless specified as an "adhesiveless" type, is actually 3 mils thick! 1 mil of adhesive is on each side of the 1 mil of polyimide film in the most standard of flex laminates.

Unlike rigid board construction notes on your drawings, it is better to add a bit more detail to ensure that you get what you want. You see, we don’t know if you know that we know what we’re doing, or, that you know that we know that you know what you’re doing… Make sense? The following sketch shows the value of a more detailed description of materials used to construct a 1 mil base, single-sided flex, with a 1 mil coverlay:

In a typical 2 layer (or double-sided) flex construction, a detailed description of materials used would look like this sketch:

Of course, flex materials are available in a variety of thickness of polyimide film, adhesive and copper. The most common types follow below:

Polyimide film is available in 0.5 mil, 1 mil, 2 mil, 3 mil and 5 mil thickness.
Adhesive is available in 0.5 mil, 1 mil and 2 mil thickness.
Copper is available in 0.7 mil (half ounce), 1.4 mil (1 ounce) and 2.8 mil (2 ounce).

If you are designing a multilayer flex or flex-rigid, the above basics will guide you into the proper construction notes by combining call outs for multiple pieces of core materials bonded together to form additional layers in your stack-up.

When adding a stiffener to your flex design, you can call them out in the same way as part of the layer stack-up. The following sketch illustrates this:

Please consult with us prior to releasing your new flex circuit design and we will be glad to help you with the call-outs. That way, we'll be sure to know that you know that we know what you really want. Thanks!