My SMT attempts failed again: And it's pissing me off

[DigiTan]

Every time I try to do a simple SMT soldering project, a bunch of random crap happens to me and I end up ruining the chip. Here's the latest attempt:

I was trying to mount an ATMEL Flash memory chip to a "surfboard" I bought from Fry's. It was supposed to be a simple job: I had an 8-pin SOIC package and all the right tools availiable. The solder was cleaned with flux and I even used latex gloves to keep from getting oil anywhere on the board. Since my first attempt failed (as usual) I tried following this guide word-for-word. Here's what happened.

First, after soaking up all the excess solder with the copper braid, I pulled out an ohmeter and found only 5 of 8 of the leads were actually making contact, even though they appeared to be soldered down under a magnefying glass.

Next, while trying to add more solder to them, my solder started mysteriously turning into a black, fouling crud that bakes onto everything and won't wash away with water. It appearantly doesn't conduct electricity either because only 4 leads worked after it appeared.

After that, I started doubting my ohmmeter, so I put both probes on one SOIC pad and discovered it had an invisible flim that obviously didn't conduct electricity unless you scratched away at it. The traces have a greyish tint to them like aluminum. I tried re-soldering the whole thing but after 2 hours of this I gave up and threw it away.


I don't see what I was doing wrong. I tried my best, did everything right and nothing worked (story of my life :lol: ). Is it possible my adhesive has something to do with the contamination? What was supposed to be a fun project ended with me punching a pillow for half an hour, ready to shoot my brains out. The experience is so frustraiting, I don't think I can stand to ever try this again. For once, I'd like to do a SMT project without going to bed angry. :evil: What the hell is happening here?

 

[Nigel Goodwin]

Is it possible my adhesive has something to do with the contamination.

What adhesive?.

Commercially produced SM boards stick the components down because they have to be turned upside down for flow soldering (and all the bits would fall off!) - you don't need to do it for hand assembly, and I would say you SHOULDN'T do it for hand assembly!.

This sounds a very simple chip to solder, I would suggest you get some liquid flux. Brush the liquid flux over the PCB pads, then carefully position the chip in place with tweezers - once it's in place tack one corner down using a very fine soldering iron bit. Confirm it's accurately in place (and the right way round! - I won't mention why I ALWAYS triple check that), then tack the opposite corner in place.

You now have the chip fixed in exactly the correct place, and the right way round (check again to make sure :lol: ), then brush the pins with more liquid flux. Next solder the pins one at a time using the fine soldering iron bit, and the minimum solder required. The liquid flux makes everything solder easily, and helps to prevent shorts across the pins.

It's really quite easy to do (don't forget to amke sure it's the right way round!), but liquid flux is what makes it so.

 

[DigiTan]

I tried using a typical super glue. One of its ingrediants is cyanoacrylate and I noticed it tends to create a white crust around the areas it hardens around. I figured it might affect the solder a little but not enogh to cripple an 8-pin connection.

Right now I have a bunch of flux that works ~okay, but its real nasty-looking, like earwax and it has tiny fibres in it that are almost big enough to pick out with tweezers.

Is there a preferred metal for the traces? I can't tell what this crud is, but it seems to have an invisible insulative coating, because I had to scratch away at it before it would conduct in some spots. What kind of cheap garbage did I buy? :?

 

[Nigel Goodwin]

No wonder it didn't work :lol:

DON'T USE GLUE!!!!!!!!!!!!!!!!!!!!!!!!!

To be honest, using super glue is rather like saying "I fitted the chip with a six inch nail and a large hammer, and now it doesn't work" :lol:

And buy proper liquid flux, it comes in little bottles with a brush in the top (rather like nail varnish).

 

[ThermalRunaway]

I'll mirror Nigel's advice. I don't know what guide you read (I didn't take the time to visit your soldering guide link) but under no circumstances should you need to use glue - especially not super glue. The problem is that even a small spot of glue will spread out to cover a huge area once you squish the flat-pack of the IC down on it, and it's my bet that this has contaminated the PCB tracks and hence why your solder attempt has failed.

Like Nigel, I tend to use liquid flux which you can buy in a small surringe package. I normally keep mine refrigerated as it works better that way. Your best bet is to first position the IC correctly so that the legs of the IC fall perfectly upon the tracks on the board and then, once you've made sure that you've fitted the IC with the correct orientation, solder pin one into place. Your soldering doesn't have to be perfectly neat at this point as you only want to tack the IC down. Then, repeat this for the diagonally opposite pin of the IC so that you've tacked the IC down using two pins, one on each side of the IC at opposite ends to each other,

Once you've tacked the IC down, run a slither of flux (don't use too much) vertically down the board at the points where the legs meat the PCB lands. Then, using your best soldering skills, neatly solder each pin one at a time. I tend to use my flat-edge soldering iron, gather a resevoir of solder onto it and then simply run my iron down the pins in one go - the flux normally takes care of the rest and you can end up with a very neatly soldered IC this way, but if you're new to it you're better off soldering each pin one at a time using a precision tip.

Once you're happy that all the pins are soldered and that they're nice and neat, you need to remove the excess flux (which is why I suggest not to use too much in the first place) I tend to use "flux-off" which comes in a spray can with a cleaning brush on the end.

Brian

 

[ThermalRunaway]

Oh and one more piece of advice, don't bother to attempt this kind of work if you're in a frustrated mood like you appear to be in now. Once you've lost your temper that's the end of it, you'll never solder an SMD component properly after that so you're better off walking away to calm down.

Bad Temper + SMD soldering = ruined board.

Brian

 

[audioguru]

The guy who wrote the guide talked about acid "plumber's" flux. You must use a rosin flux made for electronics, not for plumbing pipes. :lol:

 

[evandude]

well super glue is, as you know, extremely thin and when you glued the chip down it probably spread and got all over the pads.

I have never glued down a chip when soldering, and have never seen a reason to.

I have also never used flux for soldering a chip, although I can see the appeal and would probably use it if i had problems, I've never really had any serious problem soldering down a SMT chip.

I just put a bit of solder on one of the corner pads, then set the chip on top and melt it so the chip is held in place by that 1 pin, and solder away on the rest of it.

 

[DigiTan]

Yeah, it looks like I could have allegator-clipped the chip in place instead, but I didn't want to take any chances. So how do hold the chip in place without glue on a large board? I can't even look at it without it sliding all over the damned place.

 

[evandude]

I just told you that.

put solder on a single pad on the board, then place the chip over the pads (you can easily hold it in place with tweezers/pliers/etc
and use the soldering iron to melt that single solder pad, which then holds the chip in place.

the idea is that since the solder is already on the pad, you only need one hand for the iron and one for the chip. and also, if the IC isn't perfectly oriented on the pads, since there's only ONE solder joint, you can easily re-melt it and tweak the chip as needed, until it lines up properly.

then you can go ahead and solder the rest (generally the second pad you solder should be away from the first one, so you don't melt it and let the chip loose again)

 

[DigiTan]

But how is that possible? Doesn't the soldering still push the chip off the pads? Even with the clip, it didn't take much force at all to move it off the pads and totally f*** things up. Maybe I should also tape it down or something.

 

[Nigel Goodwin]

But how is that possible? Doesn't the soldering just push the chip off the pads? Maybe I should just tape it down or something.

"Push the chip off the pads"? - you're soldering it! - not bump starting a Jumbo jet!.

Simple step by step instructions!.

1) Place PCB on bench.

2) Brush pads with liquid flux (this GREATLY improves the ease of the job).

3) Carefully position chip in exactly the right place.

4) Place left index finger on top of chip and press lightly downwards, be careful NOT to move the chip - if you press perfectly vertically, there's no sideways force to make it move.

5) Using soldering iron with fine tip in right hand, with a SMALL amount of solder on the tip, gently touch one of the corner pins - the liquid flux should 'suck' the solder off the tip and perfectly coat the pin and pad.

6) Make sure the chip is still EXACTLY aligned, and do a similar touch on the opposite corner.

7) Remove left index finger - check it's still perfecty aligned.

8) Brush pins with more liquid flux, then solder each pin at a time.

9) Examine carefully with a magnifying glass to confirm all pins are soldered, and none are shorting out - the liquid flux helps to ensure BOTH of these.

10) Clean the flux residue off the board, using a suitable cleaner.

It's as easy as that - if you're left handed, reverse the left and right instructions above!.

 

[mstechca]

SMT is perfect for those wanting to be aggravated. Why not use DIP packages?

And with the soldering, if your solder turns weird (changes colour, etc), that is because you are mixing incorrect or random liquids with it (or are you putting it next to paper? :shock: ).

First of all, use ROSIN CORE SOLDER. Don't use Plumber's solder because it will just screw up your board in no time. I dont know how it does because I am not going to waste my money for it.

Secondly, BE GENTLE! If you are placing your anger on your soldering iron, you may run into problems. The desoldering wick should easily pick up solder. If it doesn't, then buy a new wick.

Third, FLOOD POOR CONNECTIONS! flood them to a point. If it seems like some parts aren't connecting properly, flooding a connection (or adding double the solder to a joint) corrects the problem provided it is new solder.

Four, BE EASY ON THE HEAT! While a large number of components accept a lot of heat, you should try to solder the IC's as quickly and efficiently as possible. If you are a slow solderer, then use IC sockets for all your IC's.

Lastly, DONT USE GLUE! I find glue 100% pointless here. I use my soldering iron for wax and solder. but with glue, it gets crazy. Have you read the label on your glue container? some things don't tolerate heat.

 

[ThermalRunaway]

Hmmm, I don't think I'll let you loose on any of the SSB panels I frequently work on, mstechca.

Brian

 

[zachtheterrible]

Rubbing alcohol works perfectly for removing flux.

Ive done some SMT stuff for the first time in the past couple of weeks and have had great success! I was even able to do a good job of soldering a 150+ pin chip to a "practice board" that I got from someone. The trick is to use plenty of flux.

 

[DigiTan]

Geez, rub salt in my wounds zack. :lol:

Well, as so it happens, I have one SMT surfboard left; and some obscure 78** chip I'll never need. If this doesn't work, I'm lying off this SMT crap for good. I wonder what will go wrong..

 

[ThermalRunaway]

Don't attempt it with a "what am I going to do wrong" philosophy or you're sure to cock it up. Just take your time and think about everything carefully before you do it. I've been SMT soldering for 7 years and still I take as much time to do an SMT rework now as I did when I first started. It's not something you can rush.

Brian

 

[DigiTan]

Sorry, I just seem to be having one of those weeks were something goes wrong with my projects no matter what I try. It's almost paranormal.

'Matter of fact, I'm not even gonna try. I just need to find a company that can solder this stuff for me. Besides, I need a BGA package soldered soon, and no one can do those by hand.

 

[zachtheterrible]

hey digitan, sorry about the salt in the wounds :lol:

what kind of soldering tip are you using? Maybe your tip isn't hot enough? Maybe you're not cleaning the tip right?

Did you say that you used flux? Because it makes ALL the difference in the world.

 

[DigiTan]

Eh, don't worry about it. So far, I've been pre-cleaning the iron tip with rubbing alcohol and its advertised at 40W. The flux/earwax is Kester Rosin Paste Flux "formula SP-44." One of the key ingrediants is petroleum jelly. I applied with a dental tool, also pre-cleaned with alcohol. :?