SMD noob question

[billstein]

This may really be a dumb question but wouldn't be the first I've asked so here goes. I've finally worked up the nerve to do the SMD (SMT?) learners board from Chromesphere.
I ordered a flux pen but have never used one before. Now for the noob question.
Do I coat the pads of the PCB with the stuff? Does it help the component stick onto the board so I can solder it? Do you hold the component onto the board and coat it and the board at the same time?

As you can see I am totally clueless on this one. Can somebody fill me in on the process of how you do this? 

[stevie1556]

I've done the same board, and from my experience and watching a lot of videos as I want to do more SMD stuff, I can offer this advise. It may or may not be the best way of doing it though.

It looks the easiest way it to get solder with flux already in it, but then you need to buy new solder which isn't worth it unless you do more SMD. The next best option is a flux pen (also great to use on etched boards!). I'm going to use a resistor here as an example here. Put some flux onto the solder pads of the resistor and put some solder onto one pad. Using tweezers will be easier here, so put the resistor in place and hold it down with the tweezers, and heat up to solder on the pad and it will solder the resistor in place without the need for more solder. Then solder the other side of the resistor in normally.

I enjoyed doing the SMD booster, but you do need a slightly different soldering technique.

I can't embed the video for some reason, but this video is great for learning techniques https://www.youtube.com/watch?v=b9FC9fAlfQE&feature=youtube_gdata_player

Sent from my thumbs using Tapatalk!

[chromesphere]

Hey guys,

There are a lot of different techniques for soldering smd.  Everybody has their own special approach to it.

For me, I put the flux down on the pad and then put the component ontop.  Then I apply some solder to the soldering iron.  Bring the soldering iron down to the pad, and the solder will flow off the soldering iron onto the fluxed pad.  You see, when you put the solder onto the soldering iron, the flux from the solder burns off (that's the smoke you see!).  You have to add solder to the pad, otherwise the solder wont flow.  With throughhole, you don't have this problem, because your soldering directly onto the joint.  SMD you have to transport the solder, if that makes sense?

Also, the flux will help keep the component on the board and also help to stop it tombstoning.

Really nice to hear you guys are liking the kits!  I just cant get over how small they are when you pcb mount a 9mm pot to it!  You could install one into a matchbox!
Paul

[pickdropper]

Yeah, I do it similar to Chromosphere.

1.)  Apply flux to pads

2.)  Grab component with tweezer and hold it in place on the board

3.)  Put a small amount of solder on the soldering iron tip

4.)  Touch the tip of the soldering iron to a component/pad joint

5.)  Repeat for the rest of the connections.

After everything is soldered, I often clean up the joints by adding flux to the component pads and reflowing each one very quickly.  For this step I do not add any solder to the iron tip, I just make sure the tip is clean.

If you are using a type RMA flux (like I do), you are going to want to clean (wash) the PCB when you are done.  There are two good ways of doing this:

1.)  Flux Remover - This is the best method, although more expensive.  If you have a Fry's nearby, they have good prices on Flux remover.  It's handy for a lot of things.

2.)  High-Purity Isopropyl Alcohol - This is much cheaper (you can get a bottle at Target or Walgreens for about $1).  I have a bottle of this only for cleaning boards.  I use a clean soft-bristle toothbrush (not previous used for brushing teeth) that I dip in the alcohol and then scrub the board with.  It will take longer with alcohol; you'll likely need to brush it off in 2-3 passes to get rid of the residual stickiness the flux leaves behind, but you can get it.

It sounds like a lot of work, but it's really not.  It's just a different process.  A cleaned SMT board can look really cool, too (at least I think so).  Here is a close-up of a PCB after cleaning:

 

[hammerheadmusicman]

I do it exactly the same as these guys, and it works really well.

But, i've been thinking about trying this method...


http://www.youtube.com/watch?v=0XENpPtisnM

Interesting....

Almost seems too much to be true!!

George

[pickdropper]

I do it exactly the same as these guys, and it works really well.

But, i've been thinking about trying this method...


http://www.youtube.com/watch?v=0XENpPtisnM

Interesting....

Almost seems too much to be true!!

George

No, it's not too good to be true.   That will work.  There are a few comments to mention:

1.)  I would probably avoid lead-free solder pastes for this.  It will require more heat and make your life a bit more difficult.  It's probably still possible, but leaded should be easier.

2.)  Placing the parts with a screwdriver is an unnecessary challenge.  A $2-4 pair of tweezers is indispensable.

3.)  He really applied a LOT of solder paste to some of the pads in the video.  I would try and use less than that if possible.

I've applied solder paste to bigger boards with a solder stencil and there really is an art to keeping it thin but still putting it where it needs to be.  It's an area where I really need to improve.

One of the neat thing about SMT (if the pads are properly sized) is how some of the components will self-align to the pads.  I enjoyed that part of his video quite a bit.

[billstein]

I do it exactly the same as these guys, and it works really well.

But, i've been thinking about trying this method...


http://www.youtube.com/watch?v=0XENpPtisnM

Interesting....

Almost seems too much to be true!!

George

No, it's not too good to be true.   That will work.  There are a few comments to mention:

1.)  I would probably avoid lead-free solder pastes for this.  It will require more heat and make your life a bit more difficult.  It's probably still possible, but leaded should be easier.

2.)  Placing the parts with a screwdriver is an unnecessary challenge.  A $2-4 pair of tweezers is indispensable.

3.)  He really applied a LOT of solder paste to some of the pads in the video.  I would try and use less than that if possible.

I've applied solder paste to bigger boards with a solder stencil and there really is an art to keeping it thin but still putting it where it needs to be.  It's an area where I really need to improve.

One of the neat thing about SMT (if the pads are properly sized) is how some of the components will self-align to the pads.  I enjoyed that part of his video quite a bit.

Thanks guys for the help. I was also interested in the heat gun approach, but of course this only stirred up more questions.
1. Heat guns come in many different wattages. What should I be looking for?
2. Also have many choices with solder paste. Any recommendations?