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H.J.Gelsthorpe

Surface Mount Pcb's

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HELLO ALL,

HAVE BUILT SOME CONVENTIONAL CCTS USING THROUGH HOLE COMPONENTS AND MY HOMEMADE UV BOX. I NOW WISH TO MOVE ON TO SMT METHODS, SO I HAVE SEVERAL QUESTIONS TO RAISE.

WHAT METHODS DO YOU EMPLOY TO SOLDER THE COMPONENTS, IRON AND FINE GUAGE SOLDER OR PASTE AND SOME FORM OF REHEAT?

IF YOU GO WITH THE PASTE AND REHEAT METHOD IS A SOLDER MASK ESENTIAL ?

WOULD THE ANSWERS BE DIFFERENT IF LEADFREE SOLDER/PASTE WAS USED?

IF ANBODY HAS BUILT SOME CCTS USING EITHER OF THE ABOVE METHODS COULD THEY PLEASE POST SOME PIC'S SO THAT I CAN SEE WHAT IS ACHEIVABLE

THANKS

HENRY

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I just use a small soldering iron and fine solder, it takes a bit of practice but results are ok.

Terry

post-16-1198101955_thumb.jpg

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If all you need to build is a couple of boards at a time then a proper soldering iron, .015" diameter fluxed solder, some liquid flux (to help out), and a lighted magnifying lamp, is all you should need. If you intend to build many boards, and the component count warrants it, then a plastic laser cut solder stencil, solder paste, squeegee, and small reflow oven will save your sanity. If you must use lead free solder then the required temperatures are higher, which can stress some components if you are not careful.

Sorry I don't have any decent photos to post. The one below is the best I could dig up on short notice.

post-2-1198123601_thumb.jpg

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No way... tell me you didn't solder that by hand did you? If so may I call you Master? :D

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Hi,

I just solder up by hand with a very fine tip, but have had problems doing very small chips with 40+ closely spaced legs. I also make all my boards myself with a UV box and etchant, but have found it a bit hit and miss getting really fine pads to etch properly.

I found this site a while back which gives details of DIY solder masks, through-hole plating, etc, and thought it was quite an interesting read if only for an insight into the process. http://www.thinktink.com/stack/volumes/volvi/pcbproto.htm

I guess it just comes down to how much time/money you are willing to spend on it.

Out of interest, as I understand it solder paste doesn't keep. Can anyone with experience tell me if this is true or not? I'd like to go down the oven route, but don't make huge boards and certainly don't want to be binning massive amounts of paste because it's gone off.

Si

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Well, for starters, make sure your caps lock key isn't stuck, ie. stop shouting! Other than that I think the above replies do a good job of answering your question.

Cheers,

Sander.

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I understand it solder paste doesn't keep.

That is correct. It needs refrigeration just to stay alive.

Even though I use lead-free paste, for safety I don't dare keep it in the household refrigerator. So, I bought a small electric cooler that is dedicated to its storage. The paste lives in there and only comes out when I need it. Although the stuff would never be allowed in a production environment past it's labeled expiration date, for hobby use I found that I can get about a year extra with the constant refrigeration and careful use.

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Thanks

For taking the trouble for your replies,some of the results in the pics are very profesional.

Has anybody built one of those homemade reflow ovens? or tried the hot plate method?

sorry about me shouting earlier :rolleyes:

Henry

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Daniel, I tin the PCB first then place the chip in position. Then just press the iron tip onto a corner leg until it takes to the solder on the board. Next I do the same with a leg at the oposite corner then just work my way round.

Terry

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Anyone tried hand soldering TQFP-100 with 0.5mm lead pitch? How difficult is this? Thanks.

You can do it several ways around. I usually wet two pads at either end of the TQFP, position the TQFP and fix it into place using these two pads. Now I apply some flux all around the TQFP, wet the tip of the soldering iron and let the cappillary effect do the rest when I press up against the pins and let the solder from the wetted tip wick inbetween the pin and the pad. When I'm done with all pins I again apply flux again and press the pins down with a clean flat tip and a bit of pressure, to make sure they bond properly to the pad. Alternatively you could use solder paste and a heatgun, I've done a few that way around, you do need to fix the TQFP into place prior to soldering, else it'll just float on the solder.

Cheers,

Sander.

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Daniel,

Another technique I've used is to tack two opposite ends of the IC to the PCB. I then solder the four sides with a bead of solder. I then wick away the excess with solder braid.

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Another one is to use those 45° tips that have a pit in the flat surface. As said before hand solder 2 corners of the thing to hold it in place, then put flux all around the IC, put solder in the tip's pit (acts as a little reservoir to have enough of it for all the pins), then just pass the tip on one entire side at a time.

It works with a common fine tip too, but it's a bit more tricky to get the solder amount right, if you don't put enough you'll have to stop in the middle, put more, and continue, and if there is too much it might just do a blob on the first few pins as it doesn't hold to the tip well enough.

With the appropriate tip, I've been soldering QFP208 in a couple of minutes.

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Anyone tried hand soldering TQFP-100 with 0.5mm lead pitch? How difficult is this? Thanks.

Daniel

Once you get good, with the right tip you can get just the right amount of solder on the tip of your iron and use surface tension to go down one side, leaving just the last two pins with a blob on them. A bit of solder wick can quickly remove this.

I use SMT tweasers to hold the IC in place as i tack the 4 corners, putting solder on first i find makes it very difficult to get it sitting flat and straight.

H.J.Gelsthorpe, i've built a reflow oven out of a $35 toaster oven. It has top and bottom heating elements, and i put two pieces of aluminium sheet at the top below the elements, and two at the bottom above the elements which really help smooth out the heat. Simply preheat to desired temperature (a bit hotter than the solder paste needs), place in oven, keeping the temp fairly low - so the heating elements dont start to change colour - wait for the smoke from the flux to stop coming out and then turn off and let slowly cool. Works fantastic.

Also, as far as tips go, i have some 0.2mm and 0.6mm chisel tips for my soldering iron, but i now find i do a better job with the 3mm chisel, it applies heat quicker and easier especially on ground parts, thermals or not. It also allows you to pull more solder off if you put too much on, it's all round far easier i've found.

Sander, your board looks like it was stenciled, you can see mounds of solder on blank pads, and that the IC's have floated - not been hand soldered.

Edited by Mark Harris

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