Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Minimum lead pitch to avoid bridging

Views: 3711


BR

#50919

Minimum lead pitch to avoid bridging | 5 July, 2007

This is wave soldering process and design question since most folks associated with SMT are processing mixed assemblies. We have a 3-leaded sensor (radial) with such a tight pitch always bridges. Solutions unders disussion are pre-forming and adding solder thief pads. Does anyone know if there is a design guideline for minimum pitch to avoid bridging?

BR

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#50922

Minimum lead pitch to avoid bridging | 5 July, 2007

Hi BR,

Are you screen printing adhesive to the bottom of the board? If so, have your stencil make put openings between the leads of the thru-hole part. SMT adhesive acts as a natural solder resist. So placing a small sliver between the pads works well. I do it all the time since we cannot change customers layouts, yet have to have 100% wave solder yields.

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#50934

Minimum lead pitch to avoid bridging | 5 July, 2007

We assume the three leads are lined-up perpendicular to the wave. Without taking corrective action, we know this: * 50 thou pitch will bridge * DIP do not bridge

As a corrective action that doesn't invovle a respin of the board, decrease the soldermask opening of the bottomside of the PTH for the component leads to leave 5 thou of solderable surface around the the hole. As long as you're getting hole fill, there will be enough contact between the solder and the hole wall and the the component lead to assure reliability.

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#50941

Minimum lead pitch to avoid bridging | 6 July, 2007

Krikies my friend.

A good wave ogre will be able to look at a wave and tweak in a way to minimize contact length (by slowing the conveyor speed), thus improving peelback, and lower it to the bare minimum without disrupting it's backward and forward flow.

Try this too, else the villagers in your shop may be comin' for ya with pitchforks and torches.

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#50947

Minimum lead pitch to avoid bridging | 6 July, 2007

I have to agree with the fat green guy. And with Real Chunks too. You should also run a matte mask on your board. This will help in preventing shorts.

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