Best Way to Solder-Bridge between 2 Pins

[meshtron]

I made an error on my first prototype and left the lower pink arrow pin in the picture disconnected.  Good news is the pin just above it (top pink arrow) is always high, so I just need to tie to that.  I did one of these, but it was legitimately 30 minutes of cussing and, at one point, I'd unsoldered both pins and nearly bridged to the ones below.  That time I used flux, I think that was unwise.  I have 4 more boards to do this operation to...  Also, on the right of the pic you can see a big cap - I can't access this from the top or the right (of the picture), only bottom or left.

Current plan is to try to jam soldering iron tip between the two pins (touches both at once), no flux, melt new solder on iron tip and try to get it to bridge and stay when I pull the iron away.

Besides being a better designer in the first place and not leaving a floating pin, how would you experienced guys approach this to have best chance of success?

(2nd, darker pic shows my pointiest iron tip for reference).

EDIT: Pin Pitch is 0.5mm, here is the footprint

[langwadt]

I think I'd try a single core from some thin stranded wire, L or U bend to fit the distance between pins and something to hold onto, solder and trim with scalpel or by bending the wire back and forth

[meshtron]

That's exactly what I tried the first time!  I could occasionally just get it to "stick" but could never seem to get enough heat to keep the wire and solder melted.  Maybe I'll revisit.

My soldering station is a Hakko FX888D, I turned the heat up to 850F - still seemed like (maybe because the tip is so small?) it struggled to melt the existing solder.

Also realized, at some point, the flux I'd added had "gelled" and was probably preventing good heat transfer.  I think I'll just skip flux this time, but that seems high-risk too.

[Bicurico]

Just a blob of solder to join both pins?

[magic]

Flux helps reduce bridging. Look up "drag soldering" for an extreme example.

To make a bridge, melt some solder on the tip, wait a moment for flux to evaporate, and aim carefully at the right pins.

[langwadt]

Just a blob of solder to join both pins?

that only works when you don't want it to

[meshtron]

That's the intent!  Just didn't go like I scripted it last time.  Somehow ended up with the existing solder from the pins running up onto the iron, then all of it dropping off and forming a loose ball of junk that I picked out.  Other than a nasty bout of food poisoning when I was a kid, this is the physically smallest thing that has completely defeated me! 

[IanB]

But why are you trying to bridge the actual pins when it looks like you have two huge areas of tinned copper trace on the board right next to them? I would just scrape some solder mask off the traces and bridge the gap between them with a small piece of wire (or even just a big solder blob).

In order to be successful with bridging gaps, you must first remove the solder mask where the solder needs to go. Solder mask repels solder and is designed to prevent solder bridges from forming. Scrape it off gently with the sharp end of a scalpel or craft knife.

[meshtron]

There's no copper under the disconnected (lower) pin.  There is copper pour under the top pin, but just scraped a non-assembled board clean there and confirmed it's straight to substrate on the pin in question.  Still, your point about solder mask being actively anti-bridging is good.  I'm not certain I can get in between the pins to scrape there, but it might help.

[Shock]

Solder a jumper wire across. Precrimp and tin the wire before soldering in. Use just a clean tip and flux if there is adequate solder.

[wraper]

The easiest would be to apply a bit of solder paste (with solder) and heat with hot air. With soldering iron it can be tricky because likely you will get unintentional drag soldering effect and "excess" solder remaining on the tip instead of bridging the terminals.

[xavier60]

The tip pictured in the first post looks to be not tinned properly, maybe due to operating at very high temperature. I rarely exceed 350°C
Try to bridge the pins with a strand of copper wire, using extra flux with the solder.
With that type of IC package, it can be difficult getting the tip into the corner to make good contact with the pins. The tip shape shown should work but that large capacitor? might be getting in the way.
Another option is a fine pointed tip, but most have poor thermal performance. An exception is the tip I mostly use, the T18-S4.
Yes, I know it's conical!

The flux I have settled on. https://caig.com/product/caig-rosin-flux-rsf-r80-2/

[meshtron]

That tip is unused/brand new, but my (errmmm... budget) digital microscope does weird things to colors sometimes too.

I just ordered a couple T18-S4 tips - those look better than what I've got on-hand now.  I do think poor-quality pointy tips are why I had to turn the temp up, soldering station has been a champ on everything not-this-miniature.

[meshtron]

I got a couple of them done, still not sure exactly what's happening.  With my Hakko turned up to 890F, and a new, tinned tip, I cannot get the on-board solder to melt.  If I can hold the iron just right for long enough, it at least gets "squishy" and I can push enough down from the upper pin to get to (and seemingly stay in touch with) the bottom pin.

It's almost like there's some sort of waxy stuff on the joints.  I cleaned with a brush and 95% alcohol, didn't make much difference that I could tell.  Waiting on some better tips, that might be some or most of my problem.

[bdunham7]

With my Hakko turned up to 890F

That's probably not helping.

[xavier60]

Not certain if the solder type has been mentioned. If lead free, try 60/40 leaded. If there is concern about mixing solders, wick it off and apply the fresh solder again.
There should never be a need to go anywhere near that temperature.

[magic]

With my Hakko turned up to 890F, and a new, tinned tip, I cannot get the on-board solder to melt.

Another vote for "too hot".

Heat may oxidize the tip and then solder doesn't stick to it well.
Instead, improve heat transfer from the tip to the part by applying a small drop of molten solder to the tip.

[u666sa]

You should have no problem bridging the pins using MECHANIC TY-V866 solder   

You could even grind the traces some distance away from chip and bridge them there with a blob. Flux is good, use flux. Temperature 310.

[mikerj]

When using solder to bridge a gap, pull the iron away from the joint first, followed by the solder a short time after i.e. the opposite of how you'd normally make a joint.  This encourages the solder bridge to remain.

[DavidAlfa]

Are you sure you can join VM and nSLEEP together?
Unless you're powering VM with 5V, you will damage it.
Absolute max voltage at nSLEEP is 5.5V, while VM can be up to 35V.
Edit: It's 5.5V recommended spec but can take up to 40V, you're right!

[meshtron]

Are you sure you can join VM and nSLEEP together?
Unless you're powering VM with 5V, you will damage it.
Absolute max voltage at nSLEEP is 5.5V, while VM can be up to 35V.

Yeah - I was actually very happy and surprised to find nSLEEP (as opposed to all other logic pins) can handle up to 40V!  Got lucky, and I have it working and happy on one board.

[DavidAlfa]

Now you realized it's better to break out the unused pins to testing pads (When possible) because "You never know"  .
Solder paste and hot air would be the easiest option, but nearby electrolitic might be a problem.
A thin tip should to it after some practice!

Edit: Just 4 boards? Remove the electrolytic to make some working room and that's it, I though they were a lot more.

[tooki]

There's no copper under the disconnected (lower) pin.  There is copper pour under the top pin, but just scraped a non-assembled board clean there and confirmed it's straight to substrate on the pin in question.  Still, your point about solder mask being actively anti-bridging is good.  I'm not certain I can get in between the pins to scrape there, but it might help.

It’s your own design, right? In that case, please share a screenshot of the relevant part of the PCB layout.

[meshtron]

It is - layout screenshot attached.  There's the big VIN pour to Pin 4, then the IPROPI and other lines below, but no copper (on the top layer) in the affected area.

[tooki]

Ok, so there is copper under the pin (the pad), just no copper pour or trace.

Either scrape off the solder mask between the pad and the pour (easy to do on unpopulated boards using a scalpel) and bridge with a blob, or solder a small wire bridge as others have already suggested.