hey guys!
I tried to stack some 7135…
and failed like hell!
the solder blob jumped everywhere and did bridge everything…
but NOT the wanted 2 pins which has been aligned to be soldered!
I have a new soldering iron (switchable 15/30 W) and use some Flux-filled solder; is that the problem?
also the solder “climbs” from the tip to the thicker part of the soldering iron??
I am frustrated 
that is not very different to what I tried 
maybe I try another solder…
Haha, what soldering iron and tip are you using?
Bridging gaps with solder can be tricky. I’m not sure what to tell you except “keep trying”.
Solder has surface tension. That will typically prevent it from staying at the very tip of the iron.
I find it helps to bend the legs down on the chip to be stacked, such that they are almost touching the chip on the board when it’s resting on top. This way there’s very little gap to bridge. I have a pair of hemostats with curved thin tips, I use these to hold the legs with just a very small gap showing between the stats and body of chip. Then I use my finger to push the body of the chip over, bending all 3 legs at once. I do this gently, in a couple two or three presses and have yet to break a leg off.
Since the BLF FET driver boards came out, stacking chips is something I now do rarely.
its easy see de video from the legendary master old-lumens.
ik do it with a old 30 watt solderbout .
i have set the paint sharp with the machine
You may need to back up a bit and learn how to properly clean & tin the tip first. Leaping straight into trying to bridge gaps with solder is a sure-fire way to end up frustrated and with a pile of dead parts. You'll also need some real flux, flux core solder isn't meant to replace that.
I always bend the 3 legs, use small flat/wedge tip on the iron: better heat than pointy tips, always apply flux to both bottom and top legs. First put solder on tip, apply tip first to bottom leg for a sec or two, then stroke upwards -- done a ton this way - they come out great! Back tab takes more flux, again down first then stroke upwards.
Lots of ways, but this is how I do it - this technique evolved over time w/trial & error.
I also thought I will never get it managed on my first try but it is easy after finding out how it works. I ordered a kester flux pen and very small diameter alpha solder, on my first try’s I had p solder with 2mm diameter which immediately bridged everything…
I bend the legs down 90°, I solder a bit on the stock 7135 because in stock form there is almost no solder, when the stock legs are pre-tinned I put the 7135 on top and fix it with a crocodile clip, then I put the soldering iron on the top 7135 and on the bottom 7135, after heating both up I apply the solder to the upper now hot leg not to the iron, one or two swipes up and down and voila.
And to expand on Tom's post, the reason you heat the lower leg first and for longer is because the bottom chip is already soldered to the board, and it takes more heat there to get the solder to flow. If you try to heat both the upper & lower legs at the same time you'll cook the upper one before the solder wicks onto the pins and across the gap. Both pins have to be hot enough for the solder to flow.
Even with tutorials it’s difficult and ugly at first but you will get better if you keep trying. Keep the solder wick handy so that you can start over when you’ve messed up. The chips are pretty tough.
Before about a year ago I'd never used a separate liquid flux, I figured 'well it's flux core solder, right? So why won't it stick??!'
Now, if I didn't have a bottle of flux on hand, I wouldn't even consider trying to solder anything. Life is too short for that kind of frustration.
Sorry to jump in, but posts in this thread have encouraged me to try again… So thanks!
Here’s my solution; use some POTW [Plain Old Telephone Wire] the 4 conductor type w/ 20-22 ga solid cores. Strip some short lengths. Flatten 1/4” [5-6mm] of one end. Tin the flattened wire end. Stack chips and tin the leads. Use the wire as a handle and press the flattened end to the chip leads. Touch hot iron to flattened wire. Presto! Now you can snip off the excess wire and do the next leads.
Really sucked at it forever. But when I switched to the .031 flux core solder things started changing. Then I got a Hakko 888 station and it’s now easy. I don’t use flux. I don’t use a small tip. I use a big round angle point tip that came with the iron. I’ve tried others, but that’s the one I like best. I don’t pre-tin the existing chip. But I’ve found what works well for me and when all is ON, it’s a matter of seconds to add a chip, in minutes I’ve got 16 chips on the Qlite, or even 24. 
I hold the chip in place with the tip of my hemostats or tweezers. I don’t clamp it down, just hold it manually with light pressure. And I don’t use helping hands or magnifiers. Usually solder the outer leg, either side, first, then I still use the tool to hold down on the chip just to make sure it doesn’t shift. After soldering the 3 legs, I solder the outer ground. The legs are done by picking up a very small amount of solder off the .031 solder roll, pretty much a touch of the iron. Then I press the point onto the base leg for a second or two like Tom said, then rotate the iron into contact with the stacked chip leg and slide it up. Presto! Done. I usually watch for the solder to liquify under the base chips leg before bringing the iron into contact with the stacked chips leg.
That said, there are times that many chips go easily and you feel really good about it. Then all of a sudden one leg just simply won’t cooperate and it’s bamboozling! lol
I’ve done 40 chips in a row and then had one that took as much time as all the others combined. Beats me!
But I don’t do that any more. If I want a lesser driver output I might stack 4 chips. Anything other than that I use the FET driver and control output with PWM. Like, instead of 255 for 100% I might use 190. I usually figure the percentage to get an estimate on the amperage and go from there. Most of mine, however, are full on Turbo modes at 6A or more on the top cells. I’ve found that if I want closer to 4A I can just a lesser cell.
Post # 29 in this thread (by TomE) really helped me when I was learning.
(edited to make direct link to post)
I do essentially what TomE says, but you might find it easier to break off the middle legs off to stop some bridging.
I don't presume to know your knowledge and skill level with regard to soldering, but I was a complete noob (still am) and learned a lot from this video series:
Simply from watching these videos, my soldering improved a lot. Hope that helps.