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Land Pattern Design for PLCC's, QFP's etc... in Full Radius Corners

Dreamsniper

#4614

Land Pattern Design for PLCC's, QFP's etc... in Full Radius Corners | 20 March, 2000

We used to have our PLCC,s, TSOP's and QFP's (0.65 mm pitch) Land Patterns designed in square corners. Some of our products have them designed in Full Radius Corners. As of now, with the two different designs running in our production floor, we can't see no difference between them (or any major problem in manufacturing point of view). Can anyone explain to me the difference between the 2 designs and their possible impact on the manufacturing processes. All feedbacks are welcome. Thanks and best regards, armin

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Boca

#4615

Re: Land Pattern Design for PLCC's, QFP's etc... in Full Radius Corners | 21 March, 2000

Full radius corners on pads can help with solder paste release in the stencil. The radius shape minimizes the surface area in the wall of the stencil aperture as compared to a sharp corners. When the stencil is seperating from the pcb the solder paste is caught in a tup-o-war between the surface area of the aperture opening and the surface area of the pcb pad. The pad usually wins the game and gets the solder paste, however in the corners the aperture there is more wall surface area and paste often remains in the corners of the aperture.

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#4616

Re: Land Pattern Design for PLCC's, QFP's etc... in Full Radius Corners | 21 March, 2000

Armin: Boca makes some good points about release. So, let me spin this up from a different angle ...

SM-782 suggests "full radius corners" as an alternative to the more familiar square corner pads on PLCC, TSOP and QFP. We see both also. We think they do look spiffier than square corners. We consider "full radius corners" to be a method to reduce paste volume on fine pitch devices that�s an alternative to: � Reducing stencil thickness, which (as you know) can starve "not so fine pitch devices." � Stepped stencils, which (as you know) are a pain in the gluteus maximus. � Other wackiness, such as zippers and the like.

My2� Dave F

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cklau

#4617

Re: Land Pattern Design for PLCC's, QFP's etc... in Full Radius Corners | 29 March, 2000

Hi guys;

There are a few points that should be taken in to considerations regarding IPC-SM-782 land pattern and design rules.

As a general rule all SMT reflow pattern should be in comply with SM-782 but however for special cases like :

* PLCC land Patterns:Pls Ensure that the toe and heel clearance of a PLCC device have not been reduce below IPC requirement.Any reduction in term of space would cause inspection and repair problems.

* Fine pitch QFP land patterns:As a general rule , ensure there is at least two component lead widths for the toe fillet in front of the component lead and 2 for the heel fillet behind it. (QFP land pads = 2 Comp lead widths + length of comp foot + 2 component lead widths)-IPC rules tends to have inadequate toe and heel clearances on their QFP land pattern.

* QFP pad pattern widths: Pad width should be the width of the lead of the device + 0.002".

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