Hey guys, I have my first SMD PCB on it's way. To make things a little easier I went with the 0805 resistors. I have an SOIC CMOS chip and a few supporting components. It's not huge 1" x 1.25" so a good easy start I hope.

I'm concerned that FR4 (140) might have issues with heat with reflow.

Also would like to use the "toaster oven" method for now... may purchase relfow oven in future... or build one. Do not want to use hot air.

Can you guys give me any pointers? I have been using thru hole most of my life. Have done few SMD repairs but nothing like this...

Thanks!
Charlie

 

Did you get a stencil? A stencil will make your life easier. Cut out a piece of cardboard so the PCB fits into it tightly, put the stencil over it, line it up then put a piece of tape on top of the stencil so it opens/closes like a hinge. Use a credit card or something similar to squeegee solder paste over the stencil. See the image below. I've had luck reflowing in a regular old toaster oven with a convection fan. Just set the dial to 400F, let it pre-heat, then put the PCBs in on a tray. For my boards it takes almost exactly 4 minutes to reflow a tray full of boards. I'm sure toaster oven temperature dials are far from accurate so your results may vary.

If you have paste in a syringe then with 0805 parts and SOIC chips you should be able to make it work without a stencil, but it won't be as neat and you're more likely to need to hand rework a few parts.

If you need to manually rework anything, flux is your friend, use a lot, and IMHO the paste flux in a syringe is the easiest way to go. When you're done, clean up with a brush, denatured alcohol and a paper towel. Get the brush soaked with alcohol, brush the flux on the board, then lay the paper towel over top and gently brush it with a wet brush. With a little practice your boards will come out squeaky clean.

 

If you decide to do it by hand, watch this:

 

thanks... yes I ordered the stencil with the board! I like the cardboard cutout idea. Might have to try that...

So I will need a toaster oven with a convection fan... now I know what to look for... do you see any issues with using FR4 boards in 400F is about 205 C. I wish I got the multimeter with the thermocouple to check... I will put that on my list... maybe my IR thermometer with a piece of metal on the tray will get me decent readings.

For solder paste I got Sn63/Pb37 183C. How important is cleaning afterwards???

For flux I recently got a couple of these:

 

thanks... yes I ordered the stencil with the board! I like the cardboard cutout idea. Might have to try that...


For flux I recently got a couple of these:

You want a 951 pen, no clean flux, not the rosin type. That's great flux, but it's not good for this.

What you really want, though, is probably something like Amtech NC-559-V2-TF in a syringe. It's a paste flux designed for this work.

 

I'm old school I like lead and rosin but No Clean is better!

 

I'm old school I like lead and rosin but No Clean is better!

I use leaded solder, and have 186 pen and liquid, I also have 951 pen and liquid, and, the Amtech paste mentioned above.

I strongly suggest you investigate the Amtech flux.

 

This: https://www.ebay.com/itm/Amtech-Flu...nsing-Needle-Plunger-Made-in-USA/252340246160

 

OK ordered... boy this stuff costs almost as much as gold!

 

OK ordered... boy this stuff costs almost as much as gold!

Well, maybe not quite that much. But, it's not cheap. It's great for rework, and even provides some protection from heat in that role.

 

Yeah you're right it's only $2,500 per gallon...

 

I've been using this one with great success, just for a second good option. You won't need flux for the solder paste part, only for manual rework.

For simple projects, if you're not using any parts that are very temperature sensitive, then the reflow temperature is less critical than you might think. I set my oven dial to 400F, but the thermometer inside said 450F, both were low budget items so the truth probably lies somewhere in-between. Read the datasheet for the parts you're using to see if the reflow temperature (or profile) is critical for any of them.

 

It's fine I'll never need 1 gallon of that stuff... but just making a point. I'm sure it costs a couple bucks to mix a gallon...

 

Hi Wolframore, I would really recommend using solder paste with silver in it. It really helps the solder to flow and wet the pads.
It can be a bit frustrating using a re-flow oven (i have a cheap Chinese one) as it the heating profile is a bit off the solder will not behave well and you may get poor "blobby" soldering. There is a lot of trial and error trying to get good results in the beginning.
You can however still use a soldering iron and solder paste, it works very well, and is ideal if you are building up a board in parts to test it along the way.
Using a dispensing syringe and pneumatic dispenser with the solder paste can also be a lot less mess and hassle than going for the stencil option, especially if there are not so many components, or its only a one off.

 

Thanks guys. I am just eager to get started but waiting for jlcpcb shipment to arrive. I plan on making a jig to hold the Pcb so stencil can be easily registered. The cardboard method looks like an easy idea. Maybe he I should get hot air you guys make it look so easy.

 

Hi Wolframore, the easiest way to make a jig for holding your PCB and stencil, is to use some old unpopulated PCBs. Tape them down at the far edges onto some sort of base board - one on each side of your new PCB. That way you get exactly the right height.

 

Another great idea. The stencil reminds me of screen printing.

 

Wow 32bit seems overkill for a power supply. Is this the right model? Wow that’s cheap. I like it. https://www.aliexpress.com/item/110...8b-4e94-be93-1652b068e96e&transAbTest=ae803_5

 

Practice makes perfect. If I had a choice between only a hot air station or a reflow oven, I would take the hot air station. The oven is much faster for reflowing entire boards, but at some point you will need to rework something and hot air can both be more convenient, and might be necessary depending on the part(s) that need attention. Also there are some hot air stations that have a fan in the handle, and some that have an air pump in the base. Personally I like the ones with the fan in the handle (like the ones linked above) better, the air flow is smoother which makes it easier to not blow the tiny parts off the board if you end up working with small parts. The air pump in the base type send air in pulses instead of a smooth flow.

 

The Amtech 559 is expensive, if you're paying less than $20 is almost certainly a fake product. A flux paste from any well known supplier should do the job though, and liquid flux is also fine, just maybe needs a little more cleaning up.

I don’t pay less, and I buy from reputable sources. The performance is excellent. When it comes to soldering, I prefer to pay a bit of a premium for top quality flux and solder having had bad experiences with cheaper options.

I am sure you can get good results for a bit less, but for me it’s false enconomy because I can’t be sure.