SMD with Soldering Iron - Paste vs Wire?

[kalel]

While searching for whether someone is using solder paste with a soldering iron (or would it at all work), I found this video example:



I have no experience with soldering SMD parts, and I don't plan on doing that frequently, but for the odd case when I do (maybe daring to attempt replacing a relatively large SMD component on a board), and not having a heat gun, I'm curious about solder wire vs solder paste.

[KL27x]

I have yet to think up a place where I would want to use solder paste with an iron. And I spend an inordinate amount of time doing just that. Thinking up ways to use new soldering tools. In the vid where he's fooling around with the paste, he could have done the same thing with liquid rosin flux instead of gooey lead paste. Then load the 1mm BCF tip he was using with a little bead of solder (from solderwire) and touched it to the pads.

The 1mm BCF is excellent for point to point rework with jumper wires. It's overkill for assembling a board like that. 2mm BC or a 2.5-3 mm BCF would be more efficient and plenty small enough to maneuver on that board, IMO.

I struggle to find a place to use solderpaste, even though I have a reflow oven. It's a major pain to do stencil/reflow, even, unless you have lots of small components and are doing much more than a 1 off, IMO. You need a lot of flat space and a lot of preparation and cleanup.

[woody]

Apart from the above mentioned disadvantages what I don't like about solder paste is that it is bloody expensive, needs to be kept cold and even then is usable only for a couple of months. For small or incidental projects I find it an economical and ecological disaster.

I used paste for a prototype containing a BMA180 with an LGA footprint. 24 euros of solder paste, put to use one time and a couple of months later straight from the fridge into the bin. While my 8 year old .4mm rosin core lead free roll of solder works fine every time I use it.

They should sell 1 gram packages.

[KL27x]

As long as you keep the container COMPLETELY air tight from outside oxygen, it should be able to be rejuvenated.

The problems with paste are many. First, it's a suspension, not a solution. Gravity makes the paste separate. So if the paste has separated, perhaps all you need to do is mix it up real well to make it work, again. But mixing paste causes some waste in itself.

Also, the more volatile solvents are hard to contain, so you might have to add some more to the mix.

Also, if it is exposed to oxygen it can go bad because you are building up more oxides/salts in the paste, increasing the conductivity of the residue. The acid might not be technically "active" at 40F, but that's just a buzz word. As long as the flux part of the paste is in solution, it is active... just a lot less so than when it's at 300C.

A flux like SOLID rosin is actually inert at room temp and active at 150F, or so. But in solution in a solvent, it is active enough at room temp to dissolve copper oxides and turn green with ions, if left on copper for an hour or so.

[Kjelt]

Advantage for using paste with iron:

- you have your left hand free to keep the smd part down while soldering preventing tombstoning or shifting of the part while soldering  so for fastening parts i like it very much.

- you can reliable solder 0,5mm or smaller pins if your iron is tiny enough, alternative would be fluxing and drag soldering, i never liked putting soo much thermal stress on one side of a chip package but many swear by it, so each their own.

[sasa]

I have no experience with soldering SMD parts, and I don't plan on doing that frequently, but for the odd case when I do (maybe daring to attempt replacing a relatively large SMD component on a board), and not having a heat gun, I'm curious about solder wire vs solder paste.

The easiest and the cheapest method I personally use with iron SMD soldering is to use 0.5mm soldering wire and liquid flux. Results are quite good with almost all soldering, including TQFP 100 and SOT23. The key is the flux on pads area, which as well allow to apply a bit of solder directly to the iron tip and it will provide good joints on pin(s), instead burnt out one in the wire.

[woody]

Advantage for using paste with iron:

- you have your left hand free to keep the smd part down while soldering preventing tombstoning or shifting of the part while soldering  so for fastening parts i like it very much.

I used some brass piping, a toothpick, a ballpoint spring, a 3 mm ring and a €10,- Ikea LED light to solve that problem:  a spring-loaded toothpick to hold down the component, with some extra lighting to see it all better (switched off for the picture). A wooden toothpick does not conduct heat, and if it wears out, replace it with a new one. Works great. I probably should sell them, with a monthly subscription for fresh toothpick 

Paul

[KL27x]

you have your left hand free to keep the smd part down while soldering preventing tombstoning or shifting of the part while soldering  so for fastening parts i like it very much.

This is why you use flux for SMD parts, though. After you flux the pads, you don't need solderwire, anymore. Just the solder on the tip of the iron. Holding the parts with the tweezers, you just touch the bead to the joints. It is almost exactly the same thing as using solder paste, only doing it in two parts. The peanut butter goes on the pads, and the jelly goes on the iron tip, and then you bring them together. Only two hands are needed.

This takes more steps than solder paste, technically (if you don't count warming up the paste and prepping your dispenser). But applying flux to the pads is way faster and easier than applying solder paste. You can paint it on willy nilly, taking care only to avoid getting it over ICSP pads and small connectors. You can drop the parts on, willy nilly, with no problem of smearing solder paste around. And any excess is fairly easily cleaned off afterward, if desired, in case you used way too much.

- you can reliable solder 0,5mm or smaller pins if your iron is tiny enough

You can do this the same way as above if you wanted to. Either feeding solder or using a tiny tip preloaded with solder. Or even using a relatively large tip with the right amount of solder loaded on it and using the right angle to hit only one pin. The trick is using the right shaped tip for either method. If pointy conical iron tips are all you use, solder paste is a lot more attractive. Needle pointed tips (variations of conicals, bent conicals) are not the only game in town for fine pitch point-to-point soldering. You can drag solder with the side/edge of, say, a bevel tip touching the toe/pad of one pin at a time, successively, if that is what you fancy. It's just a matter of finding the right angle. Hitting multiple pins at once just makes it faster. It takes a finite amount of time to reach the proper temp, and touching multiple pins sorta puts a couple other pins in various preheat stages, so it all blends together. In many tips shapes, the larger ones also have more bridge removal ability.

[Shock]

When you have the right sized tip good solder and flux it makes it easier, it just takes practice. This is the video you are looking for, check out his channel it shows all proper technique.

[b_force]

I solder most of my prototype boards (or small projects) just simply with a normal solder iron and solder wire.
Down to 0603 is definitely doable.

Sometimes paste (with flux) works a little faster with ics.
When you have an ic with solder pads on the bottom you need at least an hot air iron.

[rstofer]

There are several ways to get the job done and I have tried only a few:

• Drag Soldering - put flux on the pins and pads, put a SMALL blob of solder on the iron and drag across the pins.  It helps if the corners are soldered first.
• Solder Paste and Iron - put a bead of solder paste transverse to the pads, hold the component in place with tweezers and hit the pads with an iron.
• Solder Paste and Hot Plate - I put blobs of solder on the pads (I don't use stencils), place the parts and cook the board on a hot plate.  These are fairly small boards and it's important the hot plate have a flat, continuous, surface.  I just turn it to High and wait for the solder to flow.  I very carefully remove the board and set it aside to cool then turn off the hot plate.
• Converted Toaster Oven Reflow - Again, I put down small blobs of solder paste, set the components over the pads and hit the start button.  Pretty soon the board is cooked and the solder has flowed nicely.

What I haven't used is solder wire and iron.  First of all, I don't have 3 hands.  Second, it's just so much harder than using either flux paste or solder paste.  There are videos all over the Internet.  If I were just starting up, I would look into drag soldering.  BTW, you don't want a fine tip for this.  You really want a "sheep's foot" type of tip.  Something cut at an angle, larger than 3 or 4 pads and with a divot in the surface to hold a blob of solder.

Tips 1, 3 or 5 from the left work well for drag soldering:
http://www.instructables.com/id/Uses-of-Different-Soldering-Iron-Tips/

It's no big deal if you don't have one, a normal fine chisel tip will work but you don't carry as much solder so you can't get as far down the row before having to go back for more solder.  Tip 2 from the left is what I usually have installed.

Once you see how easy it is to work with SMDs, you will never go back to through-hole components.

[kalel]

Thank you. I learned that either using Paste or Wire can do the job properly, perhaps depending on the soldering tip and of course the experience of the person.