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BigEd
PostPosted: Sat Oct 06, 2012 4:50 pm 
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Here's a set of articles about using a toaster oven to perform reflow soldering:

- Reflow oven controller by Dan Strother
- Have you seen my new soldering Iron? by Kenneth Maxon
- Screen Print and Reflow SMT Boards at Home by Bob Rooks (mentions a temperature-indicating crayon)
- Surface Mount Prototype Soldering by Darrell Harmon

(BDD has previously provided one of these links, but I thought it worthwhile to start a new thread)

Dan Strother seemed set to reflow an interesting BGA-packaged FPGA design but hasn't reported on results yet.
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ElEctric_EyE
PostPosted: Sat Oct 06, 2012 11:05 pm 
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Some of those projects are more interesting than the SMT mounting "technology". Thanks for posting BigEd!
I can especially appreciate Dan's custom made oven, where he states that he did not want it to become a project, but that he needed a tool. That is skill coming in there...
I was previously looking into Infrared reflow stations for around $150US, but this may be a better option after all without worrying about IR bulb replacement after usage. Makes for a nice cheap Christmas present for me, anyway. :lol:

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Arlet
PostPosted: Sun Oct 07, 2012 7:33 am 
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I have a reflow oven that I use for small assembly runs, but I only use it when I have the stencils. I've tried the hand application method with a syringe, but I'm not impressed. It takes time (and more practice than I care for) to apply the paste neatly, and then you're still suffering from uneven application, so some pins won't make contact, and others have bridges. Checking and fixing all the pins takes just as much work as hand soldering the part in the first place. Also, the paste is expensive, and has a limited shelf life. So, I use the oven for larger runs (5-20 boards), and just do the single prototypes by hand.
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BigEd
PostPosted: Sun Oct 07, 2012 8:59 am 
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One of the PCB outfits I looked into supplied a free stencil - looks like it was PCB-Pool

(Found a single-PDF version of the Bob Rooks article here.)
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Arlet
PostPosted: Sun Oct 07, 2012 9:34 am 
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I just looked at PCB-Pool, and I noticed the free stencil was only available for small batches (under 10 boards). For larger batches, they'll charge for the stencils.

I order my stuff from eurocircuits. They don't have free stencils, but charge 135 euros for my recent 2 prototypes (118x79 mm, 4 layer). PCB-Pool charges 198 euros for the same (8 working days instead of 7). For 10 boards, PCB-Pool drops to 41 euro/board, but Eurocircuits drops to 22.50 (stencils are optional at 44 euros a piece).

When assembling prototypes, I like to build up the board gradually anyway to catch mistakes early. So I first assembly the power regulators, and check all the voltages, then I mount all the other parts one by one, and check that everything is working correctly. I wouldn't want to reflow the entire board, turn it on, and discover there's 12V on the 5V net. :)
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BigEd
PostPosted: Sun Oct 07, 2012 9:36 am 
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I wouldn't want to reflow the entire board, turn it on, and discover there's 12V on the 5V net.

Good point!
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BigDumbDinosaur
PostPosted: Sun Oct 07, 2012 7:29 pm 
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BigEd wrote:
Here's a set of articles about using a toaster oven to perform reflow soldering...(BDD has previously provided one of these links, but I thought it worthwhile to start a new thread)

Since I posted that link I had a go with this technique in building a second copy of POC V1.1 (the updated, bug-fixed version of POC V1, the original "blue wire special"). There are two SMT devices: the MAX238 transceiver, a SOIC24 package, and the 128K SRAM, an SOJ28 package. During my experiment I forgot to take pictures while the board was in the oven, the latter which is a Protoc-Silex model I got at Sam's Club, ostensibly for use in the kitchen. :wink: However, I did take a picture of the assembled board:

Attachment:
File comment: Modified POC V1 with surface mount MAX238 and SRAM.
sbc_v1.5_01.jpg
sbc_v1.5_01.jpg [ 754.51 KiB | Viewed 1543 times ]

The MAX238 is U14, located near the lower left-hand corner. The SRAM is in between the microprocessor socket and the empty EPROM socket. I used the temperature profile suggested by Ken Maxon in his article and got nearly perfect results on the first try—there was one bridge on the MAX238, which I cleaned up with solder braid. I cleaned the solder paste residue from the board using warm water and an old tooth brush. After metering all chip connections to verify continuity I decided to build up the unit and see if it would work, which it did.

Attachment:
File comment: Reflowed version of POC V1.1 undergoing initial testing.
test_setup02.jpg
test_setup02.jpg [ 725.3 KiB | Viewed 1543 times ]

Something I did to help things along with this reflow experiment was to cut a piece of quarter-inch cast aluminum tooling plate (gotten from a machine shop that is a client of ours) into a size that would fit into the oven on the rack, while leaving a one inch-or-so gap on all sides for air movement. I then placed the PCB on the tooling plate. The tooling plate slows down the rate of temperature change due to its thermal inertia, provides a solid and flat surface to support the PCB, and assures more even heating. I can't say whether I would have gotten the same results without it, but since it worked well I will stick with it for future projects using SMT devices.

By the way, something to watch for is that reflowing may close small holes. This isn't likely to be a concern with via but could be with small through-hole pads. None of my through-hole pads are small enough to pose a problem, but I did note that all .008 inch and some .014 inch via closed up.

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Last edited by BigDumbDinosaur on Sun Oct 07, 2012 8:25 pm, edited 1 time in total.

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BigDumbDinosaur
PostPosted: Sun Oct 07, 2012 7:45 pm 
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BigEd wrote:
Quote:
I wouldn't want to reflow the entire board, turn it on, and discover there's 12V on the 5V net.

Good point!

What I did to avoid such contretemps was, following reflow, visually inspect the chips with a 10x magnifier to locate any bridges or obvious signs of incomplete joints. I didn't find any of the latter but located one bridge that was easily fixed with some solder braid (aka wick). Once that was out of the way, I used the DVM to check for possible continuity between Vcc and ground (thankfully none), as well as for continuity between each chip pin and its pad (all good).

The only other advice I can offer is to be very careful in placing your parts prior to reflow and minimize the amount of time that elapses from when parts are placed until the PCB goes into the oven or other heating device. Also important to note is that the oven or whatever it is you use to heat the PCB is on a solid and level surface. As the solder paste and board reflow liquify, there's a tendency for chips to "skate" on the molten mix. If the PCB is level each chip will tend to center itself on the pads and everything will work out okay. If the PCB is not level, gravity may cause something to shift out of position, which can be a real problem if fine pitch items are being installed.

Incidentally, solder paste is conductive (of course), so cleaning up the board after reflow is essential. If you don't, it's likely your continuity tests will not succeed. As I noted above, I used warm water and a soft tooth brush for the purpose. Defluxer won't work as expected, and some brands of defluxer may damage the solder mask coating.

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Arlet
PostPosted: Sun Oct 07, 2012 8:29 pm 
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BigDumbDinosaur wrote:
Incidentally, solder paste is conductive (of course), so cleaning up the board after reflow is essential. If you don't, it's likely your continuity tests will not succeed. As I noted above, I used warm water and a soft tooth brush for the purpose. Defluxer won't work as expected, and some brands of defluxer may damage the solder mask coating.

For my last set of boards, I used Multicore LF318 solder paste. According to the data sheet, it's designed to be left on the boards. As I've noticed, it also leaves no observable residue and the boards come out looking perfectly clean, which is good, because I hate cleaning them.
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ElEctric_EyE
PostPosted: Sun Oct 07, 2012 11:24 pm 
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It seems we're all talking about QFP/SOIC packages here right? These are trivial compared to BGA. Sure there are many different/successful techniques for QFP, but I suspect for mounting BGA packages, there are only a couple of ways to do it successfully.

Who wants to master BGA soldering? I DO

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BigEd
PostPosted: Mon Oct 08, 2012 3:57 am 
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Here's another link, using a $25 oven: http://siliconexposed.blogspot.co.uk/20 ... notes.html

Also here's a $90 kit for closed-loop control of an oven, with links to schematic and a tutorial.

I think the oven approach becomes almost necessary for BGA. The Schmartboard approach is to use an array of vias to accept the BGA, such that the balls can be melted using a soldering iron on the back side. (They must be blind vias, otherwise they wick the molten ball into the PCB leaving you with an open circuit.)

Then there's hot air rework stations, and improvising that with a paint stripper:
http://hackaday.com/2012/08/27/bga-sold ... stopwatch/
(You'd have to be prepared to lose a few in the learning process.)

Cheers
Ed
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GARTHWILSON
PostPosted: Mon Oct 08, 2012 4:19 am 
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Pololu makes cheap solderpaste stencils:
http://www.pololu.com/catalog/product/446

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Arlet
PostPosted: Mon Oct 08, 2012 4:45 am 
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BigEd wrote:
The Schmartboard approach is to use an array of vias to accept the BGA, such that the balls can be melted using a soldering iron on the back side.

It looks like his tip is dirty, because the solder ball doesn't really want to stick to it. :)
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BigDumbDinosaur
PostPosted: Mon Oct 08, 2012 5:53 pm 
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Arlet wrote:
BigDumbDinosaur wrote:
Incidentally, solder paste is conductive (of course), so cleaning up the board after reflow is essential. If you don't, it's likely your continuity tests will not succeed. As I noted above, I used warm water and a soft tooth brush for the purpose. Defluxer won't work as expected, and some brands of defluxer may damage the solder mask coating.

For my last set of boards, I used Multicore LF318 solder paste. According to the data sheet, it's designed to be left on the boards. As I've noticed, it also leaves no observable residue and the boards come out looking perfectly clean, which is good, because I hate cleaning them.

Interesting. I'm not familiar with that brand of paste. I used a Kester product, which I determined by testing was mildly conductive.

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BigDumbDinosaur
PostPosted: Mon Oct 08, 2012 6:03 pm 
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BigEd wrote:
Here's another link, using a $25 oven: http://siliconexposed.blogspot.co.uk/20 ... notes.html

Sounds as though the author of that article got the cheaper version of the Proctor-Silex toaster-oven I got at Sam's Club (which is part of Wal*Mart, for those who aren't familiar with the chain). The one I purchased was around 45 USD and turned out to be pretty accurate with regards to temperature settings.

Quote:
Also here's a $90 kit for closed-loop control of an oven, with links to schematic and a tutorial.

From that site's text:

    As to be expected - we do not recommend this kit to anyone who is not familiar with electronics. The kit is entirely through-hole devices. However, when using the high-power relay, the user will need to splice into the AC power cable of the device you are attempting to control. This can be extremely dangerous causing death or dismemberment.

Now, I've certainly heard of electrocution being caused by household power, but dismemberment? :roll:

Quote:
I think the oven approach becomes almost necessary for BGA. The Schmartboard approach is to use an array of vias to accept the BGA, such that the balls can be melted using a soldering iron on the back side. (They must be blind vias, otherwise they wick the molten ball into the PCB leaving you with an open circuit.)

In the first article you linked the author made a comment about open via in the ball array. I suspect he had something like that happen to him.

Quote:
Then there's hot air rework stations, and improvising that with a paint stripper:
http://hackaday.com/2012/08/27/bga-sold ... stopwatch/
(You'd have to be prepared to lose a few in the learning process.)

I've desoldered SMT parts with my small heat gun, but have not tried soldering them that way. It seems too risky.

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