best way to hold LED in place before soldering

Thread Starter

clangray

Joined Nov 4, 2018
261
Hi, I am a novice to soldering but am making some progress lately. I am soldering LEDs to a thru-hole board and a practice PCB. In both cases I am having trouble soldering the LED into an orientation flush to the board. I am able to solder it, but after, when I flip it, there is some small amount of lead between the LED base and the PCB and thru-hole board. The electrical connection seems fine. I've tried bending them on the reverse side to both 45 degrees and in another instance, 0 degrees (flat to board). In the 0 degree example the leads end up jutting out of the solder mound to the left or right instead of straight up. How to you get by this problem? Try harder? :)

Post solder photo:

LED non-flush.png
 

dl324

Joined Mar 30, 2015
16,989
How to you get by this problem?
I hold the LED in place with my finger while I solder the first lead. Once the first lead is soldered, I check positioning of the LED. If it needs to be adjusted, I reflow the joint while using my finger to reposition the LED. Once I'm satisfied, I solder the other lead.

The leadframe on some/most LEDs will prevent them from being mounted flush with the board.

It looks like you used too much heat on the LED on the left.
 

ThePanMan

Joined Mar 13, 2020
800
First, you don't want to flush mount them. They need to cool. Soldering them flush to the board makes them run hotter. 2mm spacing is about as short as I'd go.

OK, first you tack one lead (not fully soldered). Bend the LED till it's in the position you want then solder the other lead. once the second lead is soldered you can then reflow and fill the first lead. IF the LED is just a little higher than you want you tack the first lead as close to the height you want then bend it till the second lead is at the height you want. Solder it. Then reflow the first lead and press it or pull it to the height you want.

Keep the heat to a minimum. If you have an iron with settings don't solder on the highest heat, just solder with sufficient heat to melt the solder quickly enough, flow your joint then get out of there. If you've spent more time than you should on one lead then let it cool off before soldering the second lead. 10 to 20 seconds is definitely enough to cool it to where you should be able to make the second joint.
 

Boggart

Joined Jan 31, 2022
82
The main issue with flush mounting LEDs is that they can get overheated when soldering. Cheap LEDs can be a little susceptible to heat due to the glue on the die cup which holds the chip becoming weakened. It doesn't happen a lot, but a bit of space between LED and PCB is recommended, in fact most manufacturers recommend something like 1.6-3mm. Short LED spacers are a good idea.

As for holding the LEDs in place, as others mentioned, hold the LED with your finger, solder the first lead, let it cool, align the LED and then solder the other lead. Don't move the LED while still hot as it can cause lead frames to move slightly in the LED, resulting in potentially broken bonding wires - again, it doesn't happen often, but it can.
 

MrSalts

Joined Apr 2, 2020
2,767
The main issue with flush mounting LEDs is that they can get overheated when soldering. Cheap LEDs can be a little susceptible to heat due to the glue on the die cup which holds the chip becoming weakened. It doesn't happen a lot, but a bit of space between LED and PCB is recommended, in fact most manufacturers recommend something like 1.6-3mm. Short LED spacers are a good idea.
Assuming your answer about a Bullet-shaped T-1 3/4 (5mm) style LED is true, and overheating is a problem, could you talk through how SMD LEDs survive? Even much higher power 60-120mA LEDs that require no heat sink seem to be ok directly on a PCB.
 

MrSalts

Joined Apr 2, 2020
2,767
I solder tack one lead in place. Pick up the board and gently apply pressure on the LED with my finger toward the PCB and apply soldering iron to the existing tack. It will slip firmly into place on the PCB. Then solder the other lead.
 
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Tonyr1084

Joined Sep 24, 2015
7,945
That's how I do it. Tack one lead then use the rigidity of the first lead to hold the LED in place while I solder the second lead firmly. Then I apply a full amount of solder to the first lead and push the LED into the position I desire. I also never flush mount any device that has leads. Expansion and contraction can play havoc with the solder joint and the component. As for heat dissipation - SMD's are probably better designed and held to a higher standard than cheap leaded devices. Stands to reason they have to. After all, they're typically soldered both leads at the same time and much closer to the die. Plus the introduction of RoHS typically means solders with higher melting temperatures.
 

MrSalts

Joined Apr 2, 2020
2,767
Plus the introduction of RoHS typically means solders with higher melting temperatures.
I hope the LED isn't operating near the melting point of the solder - tin/lead or all tin.
Also, all tin solder has a 50% greater CLTE than Tin/Lead solder so, I would think tin/lead solder would be a better pick for stability.
 

dl324

Joined Mar 30, 2015
16,989
PanMan mentioned this should be 2mm?
Sometimes you won't have a choice.

HP didn't even give the standoff dimensions:
1673285065144.png

Neither did Vishay (TLHB6300):
1673285107934.png

Some appear to be intended for flush mounting:
1673285220723.png
I'll check some of the LEDs I have to make sure.

EDIT: They clipped off the bars that shorted the LEDs:
rectangularLedLeadframe.jpg
These are from the days when some LEDs had silver plated leads.
 
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Thread Starter

clangray

Joined Nov 4, 2018
261
It looks like you used too much heat on the LED on the left.
From temp (680 degrees) or from applying tip too long?

How long for each step? ->Heat Lead/pad-->melt some solder from strip-->remove solder strip-->maintain heat for a little more-->remove soldering iron
 

Thread Starter

clangray

Joined Nov 4, 2018
261
First, you don't want to flush mount them. They need to cool. Soldering them flush to the board makes them run hotter. 2mm spacing is about as short as I'd go.

OK, first you tack one lead (not fully soldered). Bend the LED till it's in the position you want then solder the other lead. once the second lead is soldered you can then reflow and fill the first lead. IF the LED is just a little higher than you want you tack the first lead as close to the height you want then bend it till the second lead is at the height you want. Solder it. Then reflow the first lead and press it or pull it to the height you want.

Keep the heat to a minimum. If you have an iron with settings don't solder on the highest heat, just solder with sufficient heat to melt the solder quickly enough, flow your joint then get out of there. If you've spent more time than you should on one lead then let it cool off before soldering the second lead. 10 to 20 seconds is definitely enough to cool it to where you should be able to make the second joint.
I'm going to try the Blue-tac method. I'll post results.
 

dl324

Joined Mar 30, 2015
16,989
From temp (680 degrees) or from applying tip too long?
Applying heat too long.
How long for each step? ->Heat Lead/pad-->melt some solder from strip-->remove solder strip-->maintain heat for a little more-->remove soldering iron
It shouldn't take more than 5 seconds or so to make a joint. HP stated 260C for 5 seconds for the rectangular LEDs I mentioned.

I usually use a 700F tip and take a few seconds for each joint.
 

ThePanMan

Joined Mar 13, 2020
800
Expansion and contraction can play havoc with the solder joint and the component.
I have a TV that suffers a bad solder joint due to heat expansion and cold contraction. I have to wait about 5 minutes typically for the joint to make a good enough connection to make a clear picture. Meanwhile I have the upper left corner somewhat doubled and scrambled.
PanMan mentioned this should be 2mm?
Not specifically, but just a rule I go by. No closer than 2mm. As the afore quote and reply states, expansion and contraction can play havoc with solder joints.
From temp (680 degrees) or from applying tip too long?
How long for each step?
Quite likely too long and probably too hot. But - - -
It shouldn't take more than 5 seconds or so to make a joint.
Certainly it shouldn't take that long. But there are many factors that come into play when soldering and timing of heat application. For the average hobbiest, half a second with tin lead and an iron somewhere around 450˚F (just my preference, not a rule) (other opinions may vary), 450˚F should be sufficient. All it takes is enough time to transfer heat to the joint and for solder to flow and bond. It's called "Wetting". Wetting is like how the water in your glass sweeps up at the edges. It's the tension that pulls the water up the side of the glass just a little. When you solder, you take a small amount of molten solder (a tinned iron) to the joint. Give it about half a second (don't go timing your application because some boards may have ground or power planes that draw heat away, and longer time and often hotter irons are needed to complete the solder operation). At the half way point you add additional solder, just enough to fill the through hole and wet to the lead and pad of the joint. Once the solder has become molten and wetted the lead and pad - get the heat out of there. Don't let the lead move. Disturbed solder while it is freezing (yes, freezing. Solder freezes at high temperatures) Disturbing the solder while it is freezing makes it look grey and grainy. It's a poor joint because the molecules have been disturbed. If you get a joint that looks that way, apply a little flux and reheat the joint long enough to transfer heat fully into the solder joint. Shouldn't take more than a second. Again, different boards and conditions change everything. I've found it difficult to solder outdoors. Impossible in the winter time, the joint cools too fast for the heat to permeate the joint and flow the solder. I've even soldered on boards that had to be "Pre-Heated" before soldering simply because there was so much material to draw heat way from the joint. But I don't think you're working on anything like that. If you make a career of it then it's quite possible you'll come across that situation.
 
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