Hand soldering a SMT LED with a thermal pad is acceptable?

[treez]

Hello,
Is it acceptable to have our assembly staff to hand solder these LEDs onto our PCBs?.....

"ASMT-UWBG-NACB8" LED datasheet from avagotech (package structure shown on page 2):


there is a thermal pad right under the LED body, but I reckon I can put a line of thermal vias underneath that, and get the solderers to solder to it from the back of the PCB.

Do you believe that this is acceptable?
Inthe product, these LEDs will carry 50mA, and the PCB is just double sided 1oz copper. (the LED PCB will not be stuck to a metal heatsink plate)

 

[chuckey]

I think that holding the device at the correct orientation is the problem!. Can't you do it like it should be done, stick the thing down with the solder/flux paste and reheat it with a hot air blast. Its just that soldering from the back of the board seems very difficult as the device is so small. One way might be to make a jig which is a soldering iron bit that is the right size to cover all the pins is held vertically with the board held in some jig, so when the board is stuck in the jig, the tinned soldering iron is pushed onto the correct bit of the PCB. The operator then counts to five then tins the LED pads and places the pre-fluxed led on to the spot with a vacuum probe. Presumabley, then raises the PCB with a foot pedal to allow the joint to cool. Seems fraught to me, still you have the possibility of two connections on each of the diode, so one H/R joint would go undetected?
Frank

 

[FvM]

Why not solder the device on a hot plate using solder paste? Really easy.

 

[treez]

Why not solder the device on a hot plate using solder paste? Really easy.

do you mean smear some solder paste up the thermal vias that are under the thermal pad, and then put the pcb on a hot plate?

That sounds good, I had originally thought of soldering the pins on, then turning the board upside down and melting solder down the thermal vias that are under the heat pad..however, I reckon that would make the led un-melt its pins and fall off.
..of course, I could put "blu-tak" over the led so that it didn't fall off in this way when the board is upside down..hmmm..then again putting blu tak over the leds and resting the pcb upside down on this, may put pressure on the glass dome of the led, which is said to be a bad thing...hmmm

 

[FvM]

Yes, I mean soldering all pins at once. The main restriction is that it only works for single side assembly.

 

[treez]

would that work through a FR4 board?

 

[FvM]

Of course. I'm soldering most prototype boards this way. Some people also use a small "pizza oven" as it's sold by Beta-Layout. https://www.beta-estore.com/rkuk/order_products_list.html?wg=1

 

[pilko]

This reflow skillet method has worked perfect for me every time. The skillet only cost me $28.

https://www.sparkfun.com/tutorials/59

 

[treez]

the PCBs that are to be hand soldered with LEDs will be re-used....in other words, when the LEDs fail, the PCB will be brought back to the factory, and the old LEDs desoldered, then new LEDs soldered on by hand or in a reflow oven.
The problem will be that if the pcb's are covered in conformal coating, then that will melt off in the reflow oven.
Also, the copper pads will likely be oxidised, and so good reflow results may not be achievable.
What is really needed is hand solderable leds, but the only hand-solderable ones are the radial ones, and they never do more than 50mA.

 

[FvM]

Notice that you are talking about a different problem now (SMD PCB repair).

I was under the assumption that hand soldering refers to prototype production, because hand soldering of SMD compenents it won't be used in series production. For repair of retail products (presumed it pays at all) I neither won't expect solder irons used for SMD components. Rather hot plate preheating combined with hot air.

For production problems you'll usually ask production experts, I do too.

 

[treez]

yes, I appreciate this is not like to be too cost effective. Though with millions of people unemployed here in UK, I wonder if its better to pay a wage for people to repair the odd failed led board rather than pay the dole.
Also, it has benefits in that the copper on the pcb is recycled and put back into service without having to be melted off the board first etc etc.
These PCBs would have to be big because there are 200V worth of 50mA LEDs on them....and they use no aluminium, because having aluminium in led bulbs seems pointless unless the bulb has to be made small.
For bulbs hanging off a bayonet connector, the "bulb" can presumably be any size as long as people don't bang their heads on it.

Another problem with small led bulbs, is that they are £20 plus for 7W+.......and over here in UK, if something is that small and expensive, we like to go round pinching them from each other.
Our Travelodge hotels in UK recently fitted 50Hz magnetic ballast fluorescents in the hotel rooms......the worst for efficiency, why?...well, because nobody pinches them.

 

[Dreso12]

For soldering PCBs with small components I have build an oven following this:
**broken link removed**

I used to solder in a modified toaster oven but the results with this are much better, I can have much more power and temperature uniformity is way better.

It takes a while to build and it is not as cheap as a toaster oven but you can solder anything.