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Stencil Aperture Ratio Tests : Your input is requested, please

Mark Laing

Mark Laing

Posted May 22, 2012
6 Comments

Dear all

I am requesting your input on a hot topic within the Valor Process Engineering team this week. Namely how to calculate the aspect ratio and area ratio for stencil apertures. Now from my research so far, the most common measure of whether the aperture will release correctly has typically been the aspect ratio, however this seems to be calculated easily in simple cases of rectangles and circles. The threshold of a good aspect ratio being above 1.5. In the general case of a closed polygon, finding the “width” is extremely difficult. Therefore the area ratio, which is a function of the area of the aperture (roof) divided by the area of sides (walls) was introduced which can be calculated for any closed polygon. For area ratio a value of 0.66 or greater is required for correct release.

So I urge anyone to provide input on this topic for both aspect ratio and area ratio by posting their comments. Given what I have found on-line so far it will be interesting to see what is the most popular method for these types of calculation.

I look forward to hearing from you.

Appreciate your help.

Mark

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Comments 6

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I think integration formula should be helpful to calculate surface area of non standard shapes following link should be helpful basics for calculation : http://en.wikipedia.org/wiki/Surface_area Cross section area should be probably easier(relatively) to calculate as we can either follow same integral formula and consider only one surface Or we could divide into multiple areas and sum it up but for automation using single formula that works for surface area as well as cross sectional area is more efficient. Please let me know your thoughts..

Jay Vyas
7:52 PM May 29, 2012

Thanks for taking the time to post a comment on my blog about stencil calculations, Jay. You are correct about finding the area using an integration formula and then multiplying by the thickness of the stencil to get volume. This seems to be much easier to determine in the general case as opposed to the traditional aspect ratio. The challenge for aspect ratio is finding the width, both in terms of definition but also as a means of calculation in all cases. If you have suggestions on finding width in the general case I would be interested to hear them. Again, thanks for taking the time to read and comment on my blog. Mark

Mark Laing
11:46 PM May 29, 2012

Hello, In the IPC-7525A Stencil Design Guidelines they talk about different Aspect Ratios (page 3-6) for the the different shapes of components, but not for all components. For the smaller components (pad sizes) (0402 and smaller) only the area ratio should be considered. i think this implies that the accuracy of the area ratio is better than the aspect ratio. as for your comment on finding the width for polygons. i have also had this discussion with Pieter Allaer who sent me your draft of aspecArea ratio calculations 1). LPKF the Mfg of Laser cutting machines has a design rule check in their software that looks only for the smallest width of a polygon. 2). DEK the Mfg of Stencils are looking into this rule right now, but i have no answer from them yet. i guess it will be the same as LPKF. Stef

Stef Bos
8:59 AM Jun 1, 2012

Hi Stef Thanks for your feedback on the aspect and area ratios that I had requested. Are your stencil manufacturers considering the smallest width as the distance across the polygon or the shortest side? For example, consider a 10mm square with 1 mm corners taken off each corner. Is the distance across the polygon, in other words 10mm or the straight distance of one of the boundaries in other words 1mm? Essentially the question is what the aspect ratio is really measuring, is it the profile ratio of the shortest width of a side and the thickness of the stencil or is it the ratio of the smallest width across the polygon and the thickness of the stencil. In a lot of cases these are the same but in some examples they are different. Calculating the shortest side is very easy, finding the shortest width in the general case of a polygon is extremely difficult as I have found with numerous web searhes on the topic. Appreciate your comment. Mark

Mark Laing
3:37 PM Jun 1, 2012

Hi, DEK came up with an answer that was the same as LPKF. they both use the smallest side of a polygon maybe those guys from IPC that wrote IPC-7525 can give you a more detailed answer as for your remark : sure you can think of polygons that have a very small side, that is where the difficulty lies when you try to integrate this formula in any kind of software and are trying to detect the smallest side/width. Stef

Stef Bos
6:25 AM Jun 12, 2012

Thanks Stef So I think you are recommending that in my example below, the answer would be 1mm as the smallesside/width and not 10mm as the distance across the polygon. Is that correct? Mark

Mark Laing
10:52 PM Jun 12, 2012

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