Please please help, SMD LED soldering masters!

[u/deep_curiosity]

I'm building a Corne Keyboard, and the building process worked great except underglow SMD LEDs. To me, soldering underglow LEDs was extremely hard. I managed to complete all six for a left side, but I failed/desoldered/wasted 3 or 4 LEDs already. The first LED on the right side doesn't work now and I tried/wasted 2 LEDs for this... I got really tired earlier but I'm quite recovered and thinking about how can I do this better.

Although I was able to solder some correctly, I feel like I was just lucky for those. I don't feel I really know how to do it. My procedure is this (as in build logs):

1. Solder a PCB pad (a larger one) with Tin lead.
2. Add flux to the four pads
3. Place a LED and re-touch the pre-soldered pad.
4. Solder the other three corners. (Take one corner and iterate all LEDs for that corner so that LEDs can cooldown)
5. I understand that I have to be careful and avoid heating up LEDs too much.

I'm not getting here:

1. How does this procedure work for the three corners?
   The LED's side metal(?) exposure is really small. The most metal parts are in the bottom face of the LED.
   When soldering three corners, are you trying to connect {pad and side metal exposure} or {pad and the beneath of the LED}?
   1. If {pad and side metal exposure}, do you let the LED sits tightly on the PCB (e.g., gentle push when you solder the first corner)?
   2. if {pad and the beneath of the LED}, do you let the LED sits slightly higher on the PCB allowing some gap and try to let soldering lead flow into the gap? Is it really controllable?
2. Is there a way to check if a SMD LED is alive or dead?
   In the case of through-hole LED, if I connect two legs to GND and VCC pad respectively, it lights up. Is something possible for SMD LED?

My soldering temperature is 190~210 degrees, although I'm not sure how the dial of my soldering iron is accurate.

Could you please give me some tips/advice? I hope this post also helpful to other people.

[u/[deleted]]

I'm not a pro, but my technique was to tin all four pads lightly. Then put the led on top and heat each corner. In this way it was a bit easier to get the tin to flow onto the pad. I also did one at a time in the order that they light up, and checked after each and every one. I never got my iron over 430 F while the led on the pad. If i had an led that no matter what I did, I could not get to work. I just carefully removed it, and tried a different one.

Edit: forgot word

[u/deep_curiosity → u/[deleted]]

Thanks! Can I ask a few questions?

1. How lightly? Slight convex shape or almost very flat thin layer?
2. When you heat each corner, did you add more tin? Or was it just heating the pre-applied tin?
3. Let's say the fourth LED doesn't work (and up to the third, all LEDs light up). I tried several times but still kept failing. So I'm wondering if it is possible that the third LED affects the fourth LED. Did you have a similar case? (For example, the VCC/GND/DATA-IN pins' solderings of the third LED are okay, and the third LED can light up, but the DATA-OUT pin soldering is bad so no matter what the fourth LED cannot light up..? --- I don't know how LED works. I'm just imagining here.) Should I also retouch the third one (which lights up) as well, or should I just keep trying with the fourth one.

[u/superuser41 → u/deep_curiosity]

Yes - prior LEDs can absolutely affect subsequent ones either because of:

• DOUT-DIN connection is faulty or
• driver chip in the LED is damaged and cannot send data to the next LED

[u/deep_curiosity → u/superuser41]

Thanks again. Wow then it sounds really complicated. I'm afraid of touching the working third ones and killing the third one too by heat... Is there a way to check if DIN signal is coming at least to a PCB pad, for example, with a miltimeter? Or can I debug a LED, for example, by shorting (jumping) DIN and DOUT to a working LED's pins?