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Author Topic: Best Practice: Trimming resistor leads before or after soldering to board?  (Read 5409 times)
K0OD
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Posts: 2520




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« on: April 15, 2013, 11:57:59 PM »

Elmering 101

When mounting conventional axial lead components, such as resistors, on printed circuit boards, do you trim the leads before or after soldering them?  Does it matter?
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KD8MJR
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Posts: 1990




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« Reply #1 on: April 16, 2013, 01:54:54 AM »

I tack both sides then trim them to the length I want and after that I reflow one end at a time with extra solder that just covers over the lead tip.  It makes for a nicer looking job and I would imagine the smoother tip will help with dampening arcing if HV is present. 

Overall I would say its up to you but maybe someone else can chime in with a better method.
I don't really stick to a hard and fast rule during repairs but I do use the above method when I build kits etc.
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KA4POL
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« Reply #2 on: April 16, 2013, 02:17:08 AM »

I trim after soldering. One got to be careful using a good wire cutter so you do not apply any force on the solder point.
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G8HQP
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« Reply #3 on: April 16, 2013, 03:45:24 AM »

Trim after soldering. A slight bend of the leads will keep the component in position before you solder it. Let the joint cool before cutting (only takes a few seconds) - if you cut too early while the solder is still soft you may disturb the joint. Save the cut leads and use for zero-ohm jumpers, or extending the leads of components recovered from PCBs.

'Reflowing' a joint is a surprisingly popular way to make a bad joint which may look good on the surface.
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W1JKA
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« Reply #4 on: April 16, 2013, 03:49:19 AM »

   After soldering for easy removal,it's not unusual for me to put in the wrong resistor or wrong polarity(diode).Took me a while to learn the wisdom of checking color codes,polarities,etc. two or three times before final trimming.
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AE5QB
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« Reply #5 on: April 16, 2013, 03:50:50 AM »

The proper methodology according to uncle Sam is to tape the component in place, trim the leads using flush cutters that give a nice flat cut without a ridge in the middle, and then solder.  The solder should flow over the end of the lead.   At least that is what I learned in the U.S.N. microminiature repair class.
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AA4PB
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« Reply #6 on: April 16, 2013, 05:34:09 AM »

Place the component on the board and bend the leads a bit to hold it in place. Solder the bottom side. If you have plated-thru holes then enough solder will flow into the hole and to the top side. Then clip the leads close with a flat nosed cutter. If you want to make it look a little better you can reheat and reflow the solder on each connection - one at a time. I often do that after mounting a bunch of components.
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KA4POL
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« Reply #7 on: April 16, 2013, 05:54:44 AM »

Having the leads uncut is also supporting heat dissipation. While this is not so critical for resistors. Diodes or transistors will appreciate the advantage.
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KE3WD
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« Reply #8 on: April 16, 2013, 06:16:37 AM »

Never mind what the gummint mandated. 

Solder first, then trim. 

Sometimes you may encounter resistors with the leads already precut, too.  Doesn't matter. 

You can install several or more resistors at once, using the long leads to hold them in place by simply reaching under the board and separating the leads away from the resistor body with your fingers to hold them in place, then flip the board and solder them all at one time.  Not the way most step-by-step kit directions read, but great methodology for the experienced board populator and faster. 

If you happen to trim the leads first, not a kill, as long as there is enough lead left to effect a good solder joint to the pad.

73
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AA4PB
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« Reply #9 on: April 16, 2013, 06:46:01 AM »

I have a cutter that I tried a few times that is designed to trim before soldering. When you trim the lead it flattens the end to hold the component in place. It sounded like a good idea but it doesn't work very well because PC boards often don't have the hole size that close and the flattened end slips through the hole.

I was tought years ago in a Navy class that you never cut after soldering because the shock might fracture the solder joint. In the multiple thousands of joints I have soldered in my life I have never had that happen. We aren't building stuff for deployment on a rocket  Grin
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G8HQP
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« Reply #10 on: April 16, 2013, 08:31:53 AM »

Many years ago I once read that you should not cut before soldering with active devices, as the shock might travel along the lead and fracture the junction. A good soldered joint has more mass and will absorb the energy. Also, solder is mechanically quite soft, except perhaps when the joint has been disturbed during cooling and the solder has gone crystalline. In that case one could argue that disturbing the joint later by cutting the lead is a good thing as it might expose a bad joint.
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K8AXW
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« Reply #11 on: April 16, 2013, 09:20:59 AM »

Quote
'Reflowing' a joint is a surprisingly popular way to make a bad joint which may look good on the surface.
 

During my 50+ years of homebrewing and kit building, I've used the mount, solder and clip procedure and then had a brief go at "reflow" soldering to make the joint look better.

Quote
You can install several or more resistors at once, using the long leads to hold them in place by simply reaching under the board and separating the leads away from the resistor body with your fingers to hold them in place, then flip the board and solder them all at one time.  Not the way most step-by-step kit directions read, but great methodology for the experienced board populator and faster. 

This is the procedure I've always used and can add only one thing.  Before cutting the leads I make sure the component is flat against the PCB (resistors, diodes, elect. caps, etc. - Disc caps vertical with the same leads length in each hole)

My findings are that the "reflow" joint can hide a poor solder connection as HQP points out.  I've actually had bad solder connections hidden by a beautiful, shiny spot of solder!  They ususally look so good that it's easy to tell yourself, "That's a beautiful joint so the problem can't be there."

I also found that "reflowing" a joint for solid state devices increases the chance of ruining that device by overheating.  Reflowed joints have more solder which holds the heat longer.

I have never had a solder joint ruined by "shock" from cutting the lead.  By the time I put down the iron and pick up the wire cutter and move to the lead to be cut,  the joint has solidified sufficiently to withstand any shock.

The only exception to this that I can think of would be terminal strip joints which contains more than one lead.
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AA4PB
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« Reply #12 on: April 16, 2013, 09:55:39 AM »

I've repaired quite a number of commercial PCBs (including the dash module on my car) by reflowing solder connections to overcome bad joints. You have to be careful to add suffient heat to completely remelt the solder but not enough to overheat sensitive components. My repair work on the dash module saved me over $500 that the dealer wanted to charge me to replace it.

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K0OD
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« Reply #13 on: April 16, 2013, 11:25:54 AM »

Great info as usual from the Eham gang.

My son is facing a school EE project requiring PCB soldering. It's been awhile since I did a major project so I wasn't sure what method to advise him to use. YT videos favor the solder-then-trim method. One web source said there were different opinions on the subject. Regardless of the method advocated, no rationale was supplied.

I'm concluding that the best approach, especially for a beginner in a non critical application, is to solder, then snip. Probably no big deal either way. Having the component better anchored for soldering with the full lead is appealing to me.
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KB3TXH
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« Reply #14 on: April 16, 2013, 11:36:56 AM »

Many of the solder joints on commercial pc boards are "re-flowed".
That is because the very many components are tack soldered in place by hand,
the excess lead length is cut off, then the board is run through a solder machine
in which the entire bottom (lead side), makes contact with molten solder.
So the joints that were tacked in place, now have the solder re-flowed.

This does not cause problems unless too much heat is applied.

PC boards that are assembled & soldered by hand (rather than machine) also have the components
tacked in place. Usually just one lead to hold the component in place when the board is turned upside down.
The leads are trimmed, the untacked leads are soldered, then the tacked leads are soldered. Once again,solder
is re-flowed without problem.

At least, thats the way it was done in the defense plants, and plants that build satelites for NASA, where I worked.


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