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Reliability of U-shape appetures

Alex K.

#20578

Reliability of U-shape appetures | 2 July, 2002

Hello everybody!!!

We are going to start loading 0402s and I'm looking into all diffirent sorts of problems. From this forum I see that tombstoning is the main one.

I found article about using U-shape appeture design for 0402s. I never had stencil with one of those before, and wondoring if such shape is mechanicaly strong. Wouldnt that middle bit sticing out lifting up and cause problems or even worth damage blades??

Thanks for all your responses!!!

P.S. I have searched the Fine SMTNet Archives and didn't see anything that would asure me that there is absolutly no problem.

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JAX

#20580

Reliability of U-shape appetures | 3 July, 2002

Where did you find this article about "U-shape" apertures? I believe it is simply talking about rounded corners on the inside of a normal square aperture. ( resembling a filled in U ). It kinda follows the same ideas behind the home-plate design, with added volume and easier printability..... "printability", Is that a word...... I think not!

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Yannick

#20581

Reliability of U-shape appetures | 3 July, 2002

Hi,

WE are making some test with this kind of component and aperture and here what we think. when you use a home plate aperture it's seem that the component is not lenght enought to have a good solder but maybe is our CAD guy that didn't make a good size on the board. we didn't try a U-Shape on a resistor0402 but only on MELF and that seem to be a good thing, I suggest you to make a square aperture with a 80% opening on your Stencil or a Oblong with 80% too. You can go to 90% opening but you will maybe facing some day solder ball or tombstoning is your printing or your placement are not acurate.

Good luck

Yannick

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#20584

Reliability of U-shape appetures | 3 July, 2002

First, it's difficult to image anyone [that's fallen off the turnip truck more than once] saying 'there is absolutly no problem' about anything in our business.

Second, I have never heard of this �U� shaped 0402 aperture thing. [As someone mentioned MELF, yes. 0402, wha?] It seems like a dumb idea. It almost seems like you'd have more tomb-stoning. Can you provide us with a reference?

Third, these dang 0402 are so small that by the time you give-up the center of the pad to the inner part of the 'U' and keep good ratios, there is very little paste contacting the component.

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Alex K.

#20599

Reliability of U-shape appetures | 3 July, 2002

Thanks for responding.....

The question I was trying to ask is if U appetures are mechanicaly strong, if the tab in the middle won't stik up and damage squeegee blades? I never had stencil with such appeturs. We are just about to start some trials on 0402s so I wanted to get all the ideas.... And try diffirent methods....

The reference .... I've got an e-mail from AIM... Here is there site: http://www.aimsolder.com and link to the document: http://www.aimsolder.com/techarticles/AIM%20tech%20sheet%20Guide%20to%20Reducing%20Tombstoning%20of%20Small%20Discrete%20Components.pdf

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#20604

Reliability of U-shape appetures | 5 July, 2002

I have seen the U-shaped aperture on a customer site, being used with success. However, the problems with the stencil and the �loose� tabs where reportedly a reason to search for different aperture designs. Before this company could test their ideas of using two pads and four small circular apertures, two on each sides of the pad towards the component center, the company went out of business. Please let us all know, if this works.

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rds

#20625

Reliability of U-shape appetures | 9 July, 2002

I agree with others here who have questioned the amount of solder and reliability of the joint when using this aperture shape. I have had experience with these with MELFS, and the main issue I've seen comes when cleaning the stencil - all those little fingers grab the cleaning material and shred it. At the same time, the cleaning action damages them. As far as being a problem with damaging the squeegees - the only time this might be a concern is if you are printing with rubber blades. A normal metal squeegee blade set at 45 or 60 degree angle of attack should not be damaged at all by the small tabs from the apertures. Of course, these small tabs may be broken off by the squeegees, and then you have a problem with contamination of your paste, potential SS particles in other areas, and a stencil that has been rendered useless.

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