What else do I need from Mouser?

[chromesphere]

That's it Dave, next order, decent tweezers and flux pen...its gunna happen

[muddyfox]

Alright folks, what am I missing here?
Now that I've soldered couple dozen smd boards, I gotta ask why is the flux pen necessary here?

I've tried using it on a couple of boards (Kester 186 pen) and, apart from the extra time needed to apply the flux, I really don't see the difference in the process. I use regular 60/40 rosin cored wire (albeit a bit thinner than for TH parts) and it solders just as good if not better than TH.

Again, what am I missing?

[chromesphere]

Depends how you solder smt.  The method I use is, put flux onto pad, place down chip, apply a ball of solder to the end of the soldering iron, bring soldering iron with solder ball on the end of tip down to the chip.  The flux is there to allow the ball of solder to flow as the flux was burnt off when it was applied to the tip of the iron.

[muddyfox]

I see... that's not how I solder smd. I apply a tiny bit of solder to the pad (straight from flux-cored wire), then I grab the part with tweezers, reheat the pad and just slide the part into the molten puddle till it hits the iron (keep it in contact for a second to heat up the part as well).
Then just solder the other side as usual.
With ICs, I do this slide thing on one pin only to tack it down and solder all the others the conventional TH way.
Works just fine, apparently... never had a cold joint yet. If there's anything inherently wrong with this method, someone please take me to school!

[RobA]

The flux does a couple of things for you that are more important in soldering SMD than with PTH. The flux removes oxides that will weaken a joint and not let the solder flow as well, so even in the case where you prep the pad and then reheat with the part on, using added flux will help. You tend to have the iron on the joint for less time and there is less surface contact, so the added heat flow that flux gives is useful too. Some of this depends on the pitch size too. The really tiny leads really benefit from using more flux.

The other spot where flux really helps is when you are using solder braid to remove bridges. It makes it much easier and you can get it done faster and thus have less heat transfer to the IC.

I originally used the prepped pad technique but moved to the hold it and transfer solder to pin the corner method because I had some issues with parts not sitting flat using the other method.  It can really be a pain with MELF parts because they want to roll and the contact point is smaller.

[muddyfox]

OK... good points there. I'm not using any tiny pitch ics yet so I don't need to remove bridges but will keep this in mind.

So how does this work with melf... you have it on your desk, put solder to iron, put down solder, pick up tweezers, hunt down a rolling melf, pick it up, put on the board, all the while with solder on the tip of your iron?

Also, is it normal for the flux to evaporate this quickly? By the time I pick up the part and put it where it needs to be, all moisture is gone from the flux. Does this sound right?

[RobA]

I've heard that MELF really means Most End up Lying on the Floor .

With the melfs, I usually use flux paste in a syringe. I put the paste on the pads and tweezer the part on to the pad. I have a small coil of solder on the table by the board with the end of the solder sticking up. I hold the part with the tweezers, touch the iron tip to the end of the coil and transfer the touch of solder. I then do the second side and reheat the first side to touch up.

I have had the flux evaporate on me, but I'm getting faster and it's not so much of a problem anymore. It doesn't happen at all when you use the flux paste, but it's harder to clean up.

Check out this video, he uses several techniques and his results are crazy good.

[muddyfox]

Thanks Rob! I was already thinking of a McGyver-ish thingy that will hold up the solder for me to just pick off. 

I wouldn't think the evaporating flux would be an issue? Yes it's dry, but it's there, right? Pad cleaned and all?

[RobA]

I haven't seen anything that I think is a bad result from the flux drying a bit before I can get it all soldered. I think that there is probably some good, wet flux still between the legs and the pads. One thing that I just thought of that helps is that I only put the flux on the pin(s) that I'm going to solder at that point. When I was first starting to try this technique, I'd the whole side with the flux, pin the corner, and then drag/tack the other pins. Now I only hit the flux on the corner pin, tack it, get the solder ready to go, then flux the rest of the side and drag.

Another thing that I think really helps is getting the right tip size and shape. Counterintuitively for me, the bigger beveled tips work better. I can't get the small conical tips to work fast enough and I have much worse problems with bridging and flux evaporating when I use them.

[peAk]

This "help me fill my shopping cart" thread actually turned into a whole different animal with a lot of great information.

I haven't done any SMD stuff yet so this is very interesting. I've seen Paul's video on his J201 SMD tutorial and I guess my next question is....

are there a lot chips that are no longer being made but easy to find as SMD?

Obviously the J201 but are there others?

[RobA]

And to un-derail the thread a bit, how about DC jacks and relays.  I like some of the DC jacks that Mouser carries and they've got decent prices on the double latching relays.

[RobA]

This "help me fill my shopping cart" thread actually turned into a whole different animal with a lot of great information.

I haven't done any SMD stuff yet so this is very interesting. I've seen Paul's video on his J201 SMD tutorial and I guess my next question is....

are there a lot chips that are no longer being made but easy to find as SMD?

Obviously the J201 but are there others?

I put up my un-derail comment as you were posting this . There are more and more parts that aren't going to be done  in anything put SMD. Bunches of transistors of all types are now out of production in PTH. Lots of op-amps too. With the newer TI op-amp designs, they aren't bringing them to through hole parts at all.

[muddyfox]

Yeah, sorry about the hijack... 
*takes off skimask*

I guess I need to revisit the whole flux thing. I'm quite comfortable with my current method but now that some potential shortcomings have been brought to my attention I may have to think about it. Not that I expect any of my pedals to catastrophically fail as I never had a bad joint yet, more of a food for thought.

[sturgeo]

Up until reading this thread i've never had any desire to try SMD but now i'm feeling curious.

Is it worth creating a sticky for "order bulkers" i'm forever racking my brain trying to remember all the little bits i could add, hit the order button and then remember something obvious. Could also be useful for new builders as for example, i've been using metal LED Bezels and had never come across the LED Lens' linked earlier in this thread which will work a lot better for some of my builds.