Pad Trimming Considering the Min Body Dimensions

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   For Gullwing components, there are two options:
Either keep the pads under the body
Trim the pads so they do not extend beneath the component
Currently, when trimming, the maximum body dimensions are considered. However, this often leaves only a very small heel solder area.

Why use the maximum dimensions instead of the minimum?

The paste opening already includes a reduction, which moves the solder paste further away from the body. 

When minimum body dimensions are used, the pad remains larger, allowing more solder paste to be deposited. This results in increased solder volume and a stronger heel solder joint.

Note: IPC’s concern is that 70% of the solder joint strength is in the heel of the gull-wing lead. When you trim the heel of the pad, you effectively remove heel solder and therefore reduce the heel solder joint strength.

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GeertG Members Profile Send Private Message Find Members Posts Add to Buddy List New User Joined: 17 Apr 2024 Status: Offline Points: 7	Post Options Post Reply Quote GeertG Report Post Thanks(1) Quote Reply Topic: Pad Trimming Considering the Min Body Dimensions Posted: Yesterday at 2:46am
	For Gullwing components, there are two options: Either keep the pads under the body Trim the pads so they do not extend beneath the component Currently, when trimming, the maximum body dimensions are considered. However, this often leaves only a very small heel solder area. Why use the maximum dimensions instead of the minimum? The paste opening already includes a reduction, which moves the solder paste further away from the body. When minimum body dimensions are used, the pad remains larger, allowing more solder paste to be deposited. This results in increased solder volume and a stronger heel solder joint. Note: IPC’s concern is that 70% of the solder joint strength is in the heel of the gull-wing lead. When you trim the heel of the pad, you effectively remove heel solder and therefore reduce the heel solder joint strength.



Tom H Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 05 Jan 2012 Location: San Diego, CA Status: Offline Points: 5978	Post Options Post Reply Quote Tom H Report Post Thanks(0) Quote Reply Posted: 19 hours 17 minutes ago at 8:36am
	The gullwing terminal lead pad trimming feature is intended for packages that have no PCB standoff and the bottom of the part is flush with the PCB surface. This is when the 'A1' dimension is 0.00. This feature was added for customers that wanted to trim the pad to the 'Nominal Package Body' when the 'A1' dimension was 0.00. This was to avoid potential solder bridging under the component. However, IPC does not endorse pad trimming under the low profile component package. Their position is that no one has ever complained about solder bridging under low profile components. IPC recommends that you turn that feature off in 'Tools > Options > SMD Pad Stack Rules'. One of IPC's biggest complaints for component manufacturers that have robust tolerances on the 'L' dimension. Some 50 mil pitch SOICs L dimension is 0.40 - 1.27 mm. This makes a very sloppy pad length where the pads grow dramatically under the package. This was the cause for concern. However, todays metric pin pitch SOPs have a much more reasonable L dimension of 0.45 - 0.75. Also, if you use the mfr. recommended pattern, you'll notice that they trim the pads from under the package.
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