What's the advantage of useing a hot air rework station

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Sterling Martin
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What's the advantage of useing a hot air rework station

Post by Sterling Martin » Wed Feb 16, 2005 5:16 pm

I'm looking to jump into the SMT world, and am a little confused about the hot air rework stations. What are the pro's and con's of hot air?

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philba
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Re: What's the advantage of useing a hot air rework station

Post by philba » Thu Feb 17, 2005 1:46 am

There are actually quite significant advantages to a hot air station but I'd hold off until you have a better sense of how much SMD work you'll be doing. <p>For rework (i.e. IC removal), hot air is pretty necessary. You will want to be able to hit all the legs at once and then you can lift the chip off (or fall off it you do it upsidedown...). Just make sure you have a decent set of nozzles. Unfortunately, I can never figure out which nozzles are really necessary. This is really gentle to the PCB traces and thus you should be able to reuse the board (and potentially, the chip)<p>With solder paste, you can do hot air reflow. This is great for qfns and such but for BGA, LCCs and so on, its required. <p>However, I suggest you play around with some lower density stuff first and just use a solder station. I'd start out with 1206 Rs/Cs and SOICs. And SOT23 transistors though they are pretty tiny. You can solder all those with a regular soldering iron. The first board I used SMDs on was a TH board - all the Rs and Cs were 1206s. I put all the Rs and Cs on the bottom. It came out great. 1206s are nice because you can run a trace between the pads. Once you get a little experience, you will have a lot better sense of what you want to do.

Bernius1
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Re: What's the advantage of useing a hot air rework station

Post by Bernius1 » Thu Feb 17, 2005 3:34 am

Phil, I think this topic begs another question;
What solder do you use, & from where ? I heard liquid flux only. And what about conductive epoxies ? ((oops, 3 questios !))
Can't we end all posts with a comical quip?

Sterling Martin
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Re: What's the advantage of useing a hot air rework station

Post by Sterling Martin » Thu Feb 17, 2005 5:41 am

Alright philba, between me and no_vice, we're going to have you spinning your wheels! My questions, along with no_vice's, are:<p>1. I have a soldering iron. You've gotten me all fired up and I'm ready to waste some boards! But what type of solder do I use? I have some 60/40 rosin core that works great for other stuff, but is it the right stuff for SMD's? <p>2. I'll get whatever solder I need, at your recommendations, but now what kind of flux do I need?<p>3. Just thought that I would leave you know that I'll take your advice and start with low density stuff. With that in mind, could you please get me some kind of solder/flux recommendations? And in case you didn't figure this out already, I'm kinda ignorant, so oversimplifying is O.K.! :D

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philba
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Re: What's the advantage of useing a hot air rework station

Post by philba » Thu Feb 17, 2005 8:32 am

I don't mind.<p>Solder - I use a rosin core 22 ga. solder. Others have recommended smaller but this works pretty well for me. Its the size, not the composition that matters the most. You really dont want huge gobs of solder.<p>Flux - This is a suprisingly complex area. There are a huge number of fluxes. Others recommend liberal usage of liquid flux and I started out that way but, in general, I've found that you really don't need very much. Too much makes for a mess. I use it very sparingly or not at all. I've found that the rosin core is often all I need. If you make your own boards, you will find flux helpful with bare copper pads. Again, be very sparing... Some liquid fluxes claim to not need cleaning off but they are all sticky and attract dust and dirt even if they don't pose a long term corrosion hazard. There are lots of commercial products for cleaning flux but isopropyl alcohol works pretty well.<p>to simplify - use liquid flux in very small quantities, especially where you aren't applying rosin core solder directly. Clean your flux afterwards with IPA (isopropyl, not india pale...)<p>Solder wick. You didn't ask but its essential, IMHO. get the really small stuff (mine says #2, don't know if that's a standard). Larger makes it hard to get the braid into tight places.<p>I've never used conductive adhesives and can't help you there.<p>Phil<p>[ February 17, 2005: Message edited by: philba ]</p>

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Re: What's the advantage of useing a hot air rework station

Post by Bernius1 » Thu Feb 17, 2005 11:05 am

Have you ever tried soldering point-to-point, without a PCB ? I bought a Japanese Zero model to make a whirligig ( when the wind blows, prop-turns-generator, blinks LED 'machine guns'), and will probably have to Pt-2-Pt the circuitry in the wings (about 3/16" thk.). Also, for space-saving and copyright protection, is it feasible to stand all the SMT discretes vertically ( chips flat ), and solder a second PCB atop (Stonehenge style), E-Z-Bake or torch the backside of the board to flow the solder ? No vias, albeit RF reflections. Whaddayatink ?
Can't we end all posts with a comical quip?

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philba
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Re: What's the advantage of useing a hot air rework station

Post by philba » Thu Feb 17, 2005 12:25 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by no_vice:
Have you ever tried soldering point-to-point, without a PCB ? I bought a Japanese Zero model to make a whirligig ( when the wind blows, prop-turns-generator, blinks LED 'machine guns'), and will probably have to Pt-2-Pt the circuitry in the wings (about 3/16" thk.). Also, for space-saving and copyright protection, is it feasible to stand all the SMT discretes vertically ( chips flat ), and solder a second PCB atop (Stonehenge style), E-Z-Bake or torch the backside of the board to flow the solder ? No vias, albeit RF reflections. Whaddayatink ?<hr></blockquote><p>I'm having trouble visualizing what you are saying. How do the discretes make contact on the top? If you want to hide the circuit, why not make it as small as possible and then just cover it with epoxy. <p>You can solder little wires to SOIC8 pins and 1206s for use in a solderless BB (beats a run to RatShack). I don't recommend it but it does work in a pinch. Just don't tell anyone...<p>I think a torch would not be controllable enough - maybe ezbake reflow. The circuit doesn't sound that complex - a VR and a timer or two - so I'd probably just hand do it and then goop it with epoxy.<p>Phil

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