Just done my first board using liquid flux and I have to say its fantastic I don’t know how I went without it all this time.
I just did 3 8-bit shift registers and a PIC all SSOP20’s, all in no more than 3 minutes! Its soooo easy. Without the flux that would of taken me probably 20 minutes, a lot of swearing, and a lot of braid. It made the entire process so quick and easy I just had to comment.
If anyone else struggles with really small SMD connectors and hasn’t tried liquid flux before, grab a bottle you won’t regret it.
I expect to be doing more SMT stuff in the coming months, and wouldn’t mind some tips on how to do them more easily. Can you explain how you use flux? What’s the basic idea?
The jelly stuff comes in a syringe. For QFP devices I tack down two opposite corners, apply flux over the leads, then drag-solder them. I use liquid flux in a flux pen for other parts, wiping the dispenser tip over the pads.
I started with no flux, lead free hard 2mm solder, and an iron cranked up to 500, and still managed the job.
However over the months I have realised that leaded solder is much better to work with. I use 0.2mm leaded solder. I lowered the iron temp down to 200 and its much easier to work with now as it seems to give more even heat to board and parts.
Recently I bought the liquid flux which comes in a syringe and is very tacky. Using this made things another 10 fold simpler again, and now I have a pretty good hand at any SMD stuff I try.
The basics for ICs I use is to blob a bit of solder on the top left pad so you have a puddle on it, then holding the IC with tweezers, move it into position. Warm up the solder and slide the part onto the pad then apply some heat so it tacks down. Align the IC then tack the opposite corner pad down with a bit of solder.
Now with it in place and aligned, get the liquid flux (honestly I wouldn’t even bother soldering ICs without it now), and drop a few drips on the legs of the IC either size and it will spread all over. Now just get a healthy amount of solder on the tip of your iron and fairly quickly drag the iron tip along the IC legs with the solder facing down towards the pads. Run the iron up and down the legs a few times and inspect the result. Usually get a bit of braid/wick to tidy up any bridges. This also works better due to the flux.
As for non-IC stuff like LEDs, resistors and caps, I dont bother with flux as they are usually just 2 or 3 pins and once its tacked down the job is done anyway, but for ICs its fab.
The only thing I haven’t tried now is QFN parts… how do you go about soldering pins that are totally beneath the board? I presume it would have to be solder paste and an oven for that?
For QFN chips I extend the pads a little and drag-solder them. I use a large via for the central pad and feed solder into it whilst it is heated with the soldering iron tip.
Yes, QFN contact patches actually come up the side of the chip a bit. When I used the iron, you can just heat the pad, apply solder and it will wick in. I would start by tinning one side of the pads, place the chip and heat the pads to drop it on, then heat and feed solder to the other sides.
I use Super Solder Wire. Either that is the brand name or just a declaration of its superior ability. It says The Best Quality Solder right on the side of the spool, so you know it’s good stuff.
leon_heller:
The jelly stuff comes in a syringe. For QFP devices I tack down two opposite corners, apply flux over the leads, then drag-solder them. I use liquid flux in a flux pen for other parts, wiping the dispenser tip over the pads.
Leon
I use the same stuff, I found it works better with the IR station or hot air, whereas the liquid or stuff in a pen bubbles far too much when heated and makes my QFP packages skate across the board.
