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Topic: Double Sided SMD (Read 4823 times) previous topic - next topic

KirAsh4

Oct 10, 2012, 08:36 am Last Edit: Oct 10, 2012, 08:44 am by KirAsh4 Reason: 1
Has anyone here done their own double sided SMT work?  I've only ever done single sides with the skillet reflow method.  Obviously this wouldn't work for double sided.  So, I'm looking for ways to accomplish this.  Any particular type of oven I should start looking for?  Is it always done with epoxy on the parts that end up on the bottom on the second reflow?  Does it even have to be two reflows, or is there a way to get it all done in one pass?  Comments, suggestions, baseball bat ...

kg4wsv

I use hot air (Hakko 800 series) and have placed SMT stuff on both sides.  One wasn't really a challenge as the components that required more heat weren't back to back on the board.

In my typical double sided SMT situation, I'll have a few small caps and resistors on the bottom of the board, with most of the components on the top.  The small components don't fall off; I guess they never get hot enough to melt, or they are held by surface tension of the solder.

My boards are small, with few components on the bottom.  YMMV.

-j

CrossRoads

I have a smll 328 board with 3-4 Rs & Cs on the bottom, just a touch of an iron to install them.
Don't know what kind of glue is used to do an oven reflow for both sides at the same time if that's the way it is done.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

KirAsh4

Hrm.  The board I'm thinking of making is about 3cm x 3cm in size.  On one side it has a SOP14 IC, several 0603 resistor and caps, a SOD23 zener, and some SOT23 NPNs.  On the other side it has 3 5050 SMD LEDs and one 0805 resistor.  Hand soldering the 5050s will be rather difficult because of their proximity to each other.  And the last time I tried hot air for those, I ended up with lots of failures.

I can do a skillet reflow on either side just fine.  I just don't know how can I do both sides.  If I do the one side with the most components, what's a good way to do the other side with the 5050s?  I don't think it's safe to place the boards back on the skillet with the SMD parts down (touching the hot plate.)  Thermal shock and all.  And if I raise it off of the plate, even by a few millimeters, it might take longer to heat up and reflow the top and by then the bottom parts would be rewetted again ... I think.

CrossRoads

Kapton tape them in place & do both sides at once?
Its good to 500F.
Not to exceed temp during flow is 250C, 482F.
Guess question woul dbe hot to apply the tape without squeezing the solder paste out of place.
Perhaps pre-tin the bottom, then flux & tape the parts on while also placing the top side new?

Or get some of the glue that's used on the tape.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

CrossRoads

I have an old 4-element toaster oven I've been using, with a thermocouple to an Exar (?) brand multimeter. Just controlling the temp by hand using the temp knob - full on to get close to preheat temp, back down to turn off to let it soak and slowly ramp up a little more, then full on up to reflow phase, back down again (some, to like 425)while it ramps to ~230 & hold for a minute, then full off to start cool down, open door at 200 to complete cooling.
Have done 3 runs like that with 1, 2 and 4 small cards with 6 chips and some Rs/Cs and crystal. Only issue has been uneven application of solder paste, needed some touchup, and 1 twisted uC that had to be hot-air removed & reinstalled, and then some pin touchup by hand for not quite connected pins.
I have parts to make a reflow control card. Just need more hours in a day!
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

KirAsh4

Didn't think of tape.  I can see it being a bit tedious, as you pointed out, how to apply it without either squeezing out the paste or just moving the part out of place.  And while using glue might be a possibility, I'm still faced with the issue that I'm doing these with a skillet, which would means placing the parts right onto the hot plate.  I'm thinking I'm going to need to get an oven with top and bottom elements that are individually controlled.  If I only use one element the second time around, I can keep one side cooler, while reflowing the other ... I think.

CrossRoads

Not sure 2 elements will get the air temp hot enough to do that.  I think pre-tin, tape, and reflow could work tho, then you don't have to worry about sqeezing paste out when putting the tape down.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

KirAsh4

I see lots of folks using ovens on the net ... a lot of them with a simple U-shaped element top and bottom.  But, I've decided it's time for some experimenting so that's exactly what I'm going to do.  Thanks for the suggestions.

Grumpy_Mike

In industry they are always done one side at a time, and once again over a solder wave for the through hole components. It is a special epoxy glue they use on the components.
However I would be tempted to hand solder the least populated side second.

KirAsh4

Yeah I thought about that Mike.  The issue is that the 5050s are very close to each other, too close to get a thin soldering tip between them.  I suppose my other options are to enlarge the PCB and either get all the components on the same side, or enlarge the PCB to add more spacing between the LEDs.  Neither are particularly desirable for this project.  :/

Grumpy_Mike

Not sure what the pitch of a 5050 is but it can be hand soldered. Simply flood the connectors with solder then remove it with solder braid. It works on the finest pitch devices I have seen, that is 0.5mm.

KirAsh4

Individually yes.  But not when they are in very close proximity of one another.  There are 6 of them very close together.  That's why I said, one (less desired) option is to enlarge the PCB and put more space between the 5050s.

Jantje

you probably heard about drag soldering.
But in case you hadn't now you have
Best regards
Jantje
Do not PM me a question unless you are prepared to pay for consultancy.
Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -

Graynomad

Use a hot air rework station to do the LEDs manually.

_____
Rob
Rob Gray aka the GRAYnomad www.robgray.com

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