I'm building a batch of boards with mechanical relays. I decided to do dip soldering with a 35cm*30cm lead-free solder bath. I start a board at about 30 degree angles and finish at -30 degree angles with a duration of about 6 seconds. I occasionally get a relay that doesn't consistently fire. If I tell the relay to actuate, it may fail to actuate. But if I press on the relay or flex the board, it may actuate. So my way to fix is to reflow solder at every relay pin with an iron, kind of defeating the dip soldering. I wonder if someone has advice for me regarding this. Other components I dip solder, such as pins and terminals don't suffer as much because they are not electro-mechanical. I use thin profile relays. Maybe larger relays are better against this when there's some stress from soldering? Thanks.
Thermal stresses, difference in surface characteristics of the pins used (some components/pins just wet more readily than others) - something along those lines. I'd lean towards the latter more so than the former. Is there anything you can do with optimizations in terms of flux application?
Thanks. For solder, I use Kester K100LD bar solder. I use a spray bottle with Kester 959T flux to spray the whole under side of my board. Maybe I can try dipping my board in the flux? I Kapton tape parts of the board so thru holes aren't all filled with solder. During dipping, I could feel the board warp upwards so the opposite sides of the boards aren't touching the solder at the same time so I have to rock/stir the tongs that I use to hold the board to make sure all parts of the board are dipped. Otherwise the holes on the outside may not fill. If I dip for too long, SMD parts will start getting desoldered. If I reduce temperature, I get some crusty results.
What I do notice with the relays that have issues is:
The solder looks good but if I reflow with an iron, it seems to be just a tent of solder between the pin and the via ring so the tent collapses into the via hole when I reflow that pin. With normal pins (same relay parts but not having issues), if I reflow with an iron, it's a solid cone of solder, not just a tent, so it doesn't collapse into the via hole because the hole is already filled.
This suggests to me that the pins aren't wetting properly. They may have grease or oxidation on the surface that the flux somehow doesn't get rid of. You could try dipping the pins of the relays in an organic solvent followed by a mineral acid to clean them. But I'd start by inspecting the pins very closely and manually wetting them with solder to see if you can reproduce the problem. I bet it's some kind of contamination/oxidation that results in poor wetting.
I'll check but I usually use the parts not long after I order them from digikey. Since the pins are very easy to wet with an iron, I kind of doubt that they were oxidated. Maybe my spraying of the flux isn't enough. I'll try dipping my boards with flux in a plastic box to see if I can get better. Today when I was dip soldering a different board with lots terminals, the 3.5mm pitch terminals were all fine, but almost all of the 2.54mm pitch terminals had one jumper. I tried to spray at different angles several times but maybe that's just not enough.
@JohnRob I was secretly hoping that pre heating wasn't needed. I guess another few hundred dollars worth of equipment at least and a much slower throughput due to the pre-heater size limitation. Currently I have some metal rails to stage a bunch of boards or panels. I then place components, and then pick up from one end with tongs and dip. I guess I will be limited by the size of the preheater now. It's such a pain to do production. The solder bars are already about two grand! Hungry hungry solder pot! Sigh!
Hi @TomGeorge The solder pads are from a library part. One relay has 61mil pad diameter with 41mil diameter drill. The other relay has 71 mil diameter pad with 51 mil drill. Neither is big. My 0.1" headers have 70 mil pad diameter (octagon) and 40 mil drill diameter, so more area to wet. Maybe I should increase the pad size to give more area for wetting? The relay pins are thin blade type, not circular or square pins so there's enough space to wet into the hole. I am guessing that larger pad area will help with wetting. May not solve the stress issue but certainly don't have to dip twice when several pins don't solder
There a lot of guides and component suggestions as to the soldering "profile". My guess there is a lot of help there.
Knowing nothing about your board except it has some relays, I'll guess that even if the relays were not an immediate problem there is likely some longer term failures due to stress.
I'm not sure what profile there are for dip soldering. I understand how a reflow profile works for SMD components. Do you mean pre-heat profile for a pre-heater? Could you be more specific what I should be looking for? I want to avoid any type of failure. Thanks.
The board just needs to be wet with the flux and the flux needs to be designed for lead-free solder. Then after your soldering is done and cooled, scrub the board to remove any excess flux as it will continue to etch the metal parts.
So maybe an expensive preheater with exposed heating coils/lamps and a framework on top to hold one board?
Those flat-bed heaters will be no good, will they?
This seems to be overtly expensive and slow, place board on preheater for a few minutes, move over to dip solder, maybe place on a rack to cool off. I'm not sure if this will save time from hand soldering or not. I must be missing something.
Here is what I'm doing right now, focus on completing each step for all boards:
Prep all boards with Kapton tape covering parts that don't need to be soldered (could take 20 minutes)
Place thru hole components to all boards that my metal rack can hold, say 16 panels (takes an hour or so depending on component count)
Take one board with tongs, spray with flux, dip, place under a fume extractor for a few seconds
Look at the bottom of the board, place on surface to cool, do the rest
Maybe I should have done, focus on one board?
Prep board 1 with kapton tape, place components
Place board 1 on pre-heater
Prep board 2 while board 1 is getting pre-heated
Remove board 1 from pre-heater and place board 2 on pre-heater
Spray board 1, dip solder, cool on rack
Repeat with board 2 and 3.
Any advice on work flow? I could possibly use a reflow oven to preheat and hold say two or more boards in the oven at a time but they only heat from above not from below.
Thanks. The flux as I pointed out is Kester 959T which is no-clean non-corrosive and designed for lead-free wave soldering. I think I got close enough to what I need in terms of flux. I'll try dipping to get better wetting.
Sorry I can't help too much with the details. I was more engineering and reliability.
I would look for an IPC spec. I know they are pricey for a small business, however if you look hard enough you can probably find an older version you can download.
I'm sorry but I'm having a hard time following you @JohnRob . If you meant a spec for preheating a board, I would just use what I use in my reflow oven for SMD. It's for removing stress in the board so a slow ramp of a few minutes to 170DegC would do. But then the next board will not have this profile unless you flush out the hot air and cool the preheater and restart. Anyway, this sounds like the only proper way is to have a conveyor multi-stage oven to preheat the boards and a wave soldering machine to do the soldering. Dip soldering must be a cheap alternative i.e. quality is reduced. So I'll skip a "profile" and just place a board in a reflow oven set to 170DegC and wait for a few minutes and call it good. To keep the board inside the reflow oven off the bottom of the drawer, I will sacrifice a board to make it into a stand. If this makes board warping less visible, then it's better than not preheating.
That may be so, but your original posting relates to certain components not getting properly soldered. That tells me the flux is not cleaning the relay contacts. You need a more aggressive flux!
