Trimming leads

Casimir,

The Q and other lead trimmers work extremely well if properly used. Blade life is directly proportional to to the height of the cut. There are a few factors that are very important in trimming. You are working with a solid carbide blade which is like glass. Every time you trim a board you put more nick's on the blade edge. After a while, the blade begins to look like a saw and wants to push the lead over instead of cutting it off. If you stay close to the solder fillet, it will take longer for the blade to push the lead over even though the blade is still getting nicks in it. Here are some tips for successful lead trimming.

First, the board must be held flat by vacuum with many standoffs unless it is smaller than 4" wide and held by Q2G mechanical fixturing. Even boards as small as 4" X 4" should have at least one standoff in the middle. An ideal trim height is just above the solder fillet or .020"-.030". A realistic maximum lead length is about .040". Any longer than that, the blade has a tendancy to push the lead over. Don't try to cut header pins or connector leads. These will wipe out a blade almost immediately.

When you trim a board close to the solder fillet, it is very important that the board is flat and properly fixtured. Be careful that you don't get any leads from the last board trimmed between the fixture and the board. I always had people use a small paint brush and brush off the fixture after every board trimmed.

As a rep years ago for Q Corp, I sold about 150 of their lead trimmers. I then sold hundreds of cut and clinch component locators for Contact Systems and found out that you can't trim clinched leads. YOU CAN'T TRIM LEADS THAT ARE ALREADY CLINCHED. There is just too much cross section. The clinched cut lead looks like a tear drop and usually folds over before it cuts all the way off. Q Corp sells a motorized brusher and hand brushes that some times help with occasional "fold overs" but the blade life is poor if you cut too long or clinched leads.

I have a lead trimmer for sale made by a company called Microscan in Washington State. It has a built in sharpener on the machine so you don't have to change blades so often. My experience with this machine is that you should lightly dress the edge of the blade after every 3 or 4 boards trimmed. This will allow you to possibly trim leads to maybe as long as .060" (and possibly above closely clinched leads).

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Casimir: A comment and a couple questions:

* Have you considered a machine that forms component leads to "self-clinch?" With this forming components snap into hole in the boards with the proper finished lead length and requires not trimming after soldering. Sources are in the SMTnet archives (lead forming). Bill will sell you one. * How are you soldering these components before trimming? * What level of mechanical shock/stress does trimming with dull blades have on solder connections? This process appears to be an attempt to pull the component/pads/barrel through the board.

Good luck.

Dave F

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