How to resolder an inductor to PCB?

[weriw]

Hello, I would like to solder inductors that has fallen away back to their place on PCB and would like to ask for some advices.

I have a power board (DC/DC converter and battery charger) from an old laptop and 3 inductors (all 10 uH) have fallen away. Please, take a look at the photos. I'd like to reattach them back to the board (either these 3 or 3 new ones). How should I proceed? I searched for advices about soldering surface mounted component and found suggestion to first solder one pin and then the second one. I don't think that is possible in my case, since pins extend less than 1 mm from the bottom of the inductor.

- Could I solder both pins at the same time by heating two small drops of solder on PCB and then quickly and gently pressing the inductor onto them?
- Do I need to clean the board and the pins? I believe there was solder paste used, but I am not certain.

[Ian.M]

Short of reflowing the whole board in a temperature profile controlled reflow oven, there's no hope of fixing that properly without a hot air rework station, preferably with a board preheater as well.   The pads of both the inductors and board must be cleaned, probably best done by re-tinning them with solder using an ordinary iron then brush and wick off as much solder as possible, then finally scrub with IPA or flux remover using a Q tip.  Then you'd be ready to apply solder paste, and position and reflow the inductors.

If there's enough clearance in the housing, you might be able to get away with bodging it by hand-soldering very short jumpers of magnet wire between the board pads and the inductor pads then squishing the inductor into place on a blob of neutral cure silicone sealant or other non-corrosive resilient heat resistant glue.

[weriw]

Thank you for the suggestions. Would it make sense to turn the inductors on their side (rotated by 90 deg.) then solder the short jumpers you suggested?

[Bud]

Seems to be a sizeable inductance value, just mount in a convenient way and solder with wire jumpers.

[Ian.M]

You really should avoid mounting them on top of other parts of the circuit where their magnetic field may induce voltage in nearby tracks, possibly disturbing normal operation.

Due to the close packed components and tracks, there's nowhere good to mount them sideways and still keep the jumpers short, and the PCB pads accessible for soldering, hence my 'squish down in place' suggestion.

[weriw]

Thank you for advice! I'll see what I'll be able to do.

[Kleinstein]

They are inductors not used for there exact value anyway, so a little more inductance from jumper wire would not hurt that much. There is some stray filed on the to side of the inductors. It would still be worth a try to mount them at an angle and maybe bend them back to just a little over there original position.

Re-flowing  the whole board or just locally cold be tricky and damage other parts. Those inductors look like difficult to solder parts with the pads well hidden under a large mass. Poor solder quality during manufacturing could be reason for the fault.

[Gyro]

What possible excuse could a manufacturer have for using those stupid heavy SMD inductors right next the a bunch of T/H electrolytics? 

[Ian.M]

Unless its double sided, and densely populated on the bottom, *NO* *EXCUSE*!  Even if it was a legitimate design decision to use SMD inductors, it was stupid to use ones without accessible pads to aid reflow, and to not have used SMD adhesive.

[David Hess]

Clearance is limited by the height of the tallest capacitors so I would mount the inductors on stilts so that a pencil soldering iron can reach under to solder them to the board.  Depending on the environment, they will then need to be physically restrained to prevent vibration and shock from breaking the solder joints.

[weriw]

Consider conductive epoxy or low melt (SnBiAg) solder. They should make your life easier if you really have to solder them back.

I was thinking of using the Pb-Sn solder with lowest melting temp (~180 C). Is SnBiAg much better?  I can't find it locally.

[weriw]

Unless its double sided, and densely populated on the bottom, *NO* *EXCUSE*!  Even if it was a legitimate design decision to use SMD inductors, it was stupid to use ones without accessible pads to aid reflow, and to not have used SMD adhesive.

I would say this qualifies for densely pupulated.



I have one more question: I guess I should really be careful not to get solder onto the connections to the components on the bottom side (circled blue in the image). But it's safe to get solder onto the ones marked with white circles, right? Because they are already connected to one side of the inductor through PCB.

[Ian.M]

Just solder neatly to the pads - there's no need to get solder on the vias! 

There's also no need for low temperature solder unless you are attempting to reflow the inductors in place as originally assembled.   If you do use Bi containing low temperature solder, be aware its typically much weaker than 'regular'  SnPb or SnCu/SnAg solders, so SMD adhesive under the inductors to reduce the stress on the solder joints would be essential for reliability

If you want a nice neat job, without reflowing it, I suggest starting by soldering short insulated jumpers to the inductors, slightly over one inductor diameter long, parallel, sideways (same side),  at right-angles to the center-line through both inductor terminal pads.  Clean all flux residue off the joints and dry thoroughly. Then stick a disc cut from 3M foam VHB tape over the inductor base and the jumpers, leaving the release paper on, and bend the jumper leads up at right-angles at the edge of the tape.

You should then be able to easily solder the other end of the jumpers to the inductor pads on the PCB with the inductor lying on its side, clean up any flux residue, then peel the release paper off the 3M VHB tape and fold the inductor down into place.

[weriw]

Thanks! Sounds like a neat idea.