Extreme Surface Mount Soldering

So you're thinking of building a super-widget and wanted to use the latest QFN / MLF (micro lead frame) package parts. But then you actually LOOKED at one, realized how tiny it really is (Picture 1), and decided no way can I solder that! Think again! This instructable will show you how to design and build a circuit using truly tiny SMT devices. Not only will I show you how to build your own Hot Plate Soldering System capable of soldering the tiniest surface mount components using lead-free solder, you will learn to design the footprints, stencil on solder paste, and solder the components. I'll also point you to cheap sources for design software, PC boards, solder paste, and stencils.

This instructable is about two things: How to build and use a Hot Plate Soldering System, and general guidance on how to design circuit boards using surface mount parts. All the information you need for successful surface mount design and construction. Throughout this Instructable, many sources will be cited: like Newton, I truly stand on the shoulders of giants! (That's another way of saying others have already worked most of this out.) What I'll do is explain the methods I've used successfully to build tiny surface mount circuits. The focus is on specifics, not generalities. I believe the method and tools I'll describe are the most economical available which will yield satisfactory results.

So let's get started. Successful design and construction with those Extremely Tiny devices you've been dying to use is just an Instructable away!

UPDATE February 2010: Closed Loop Control has been added! Have a look at this Instructable.

Step 1: Order Parts

Order the parts to build the Hot Plate Soldering System. Here is the Parts List with Digikey Part descriptions and numbers:

Parts for the Pulse Width Regulation Unit: (If you plan to use a Ghetto Development System, then you already have U1.)
U1 IC MCU AVR 2K FLASH 20DIP ATTINY2313-20PU-ND
U2, U3 IC I2C I/O EXPANDER 568-4236-5-ND
SW1, 2, 3, 4 SWITCH DIP EXT RCKR UNSEALD 4POS GH7170-ND

Parts for the AC Control Unit:
MOC OPTOCOUPLER TRIAC-OUT ZC 6-DIP MOC3042M-ND
SCR1, SCR2 NON-SENSITIVE GATE 20.0A 400V S4020L-ND
R3 RES 180 OHM 1W 5% METAL OXIDE 180W-1-ND
R1, R2 RES 1.0K OHM 1W 5% METAL OXIDE 1.0KW-1-ND
D1, D2 DIODE GEN PURPOSE 50V 1A DO41 1N4001FSCT-ND
Heat Sink HEATSINK TO-220 W/PINS 1.5"TALL HS350-ND

For the Pulse Width Regulation Unit you'll also need LED1 and LED2, resistors R3 and R4 (470 Ohm), pull-up resistors R1 and R2 (3.3K Ohm) for the I2C bus, and R5 (470 Ohm) between the MOC and the Pulse Width Regulation Unit. These can be from the junk box or Radio Shack and the values aren't real critical. You'll also need some perf board. Get the kind without any metal on it for building the AC Control Unit. One other item is some silicone heat sink grease  also from Radio Shack if you don't have any. Finally, you need S1, a momentary contact push button, and you should have C1, a 10uF cap.

Comments

[salmansheikh]

What precautions and is it possible to do this AC control unit on single-sided copper pcb instead of the perf board?

[doctek]

Yes, a single sided board is certainly possible.

Precautions:
- Make sure the traces can carry the current.

- Make sure the traces follow the spacing rules for the AC voltage you use.

Note that the wire and insulation used on the perf board version takes care of these problems.

Probably an even better solution is to simply buy a solid state rely off Ebay. Suitable units are available for less than $10.

[dscanner]

Xcellent project, although just starting. Question on the hot plate. I have seen a lot of different versions similar to the Elite, most are rated at apprx 1000W/115vac. Is this wattage adequate to get to 450 degrees per discussions in this instructable? And what temp does the solder paste actually melt at? I am actually seeing higher temperatures for some types of solders.

[doctek]

Great question! Your basic hot plate (any one will do) will easily glow a dull red at high heat. That's about 900F (480C) - well beyond melting point of most any solder intended for electronics use that I know of. The exact melting temperature of a particular solder depends on the exact metals used in it. Most melt in the range of 180C to 220C; consult the data sheet for your solder for the specific temperature. Know that the temperature you want to use is slightly above that value by 20C or so to allow for heat transfer through the circuit board.

BTW, the easiest way to convert from F to C or C to F is to add 40, multiply by 9/5 (C to F) or 5/9 (F to C), then subtract 40.

[salmansheikh]

May I use 220 ohm resistors instead of the 180ohm?

[salmansheikh]

Can you use an Arduino to get your PWM and not need the I2C expanders?

[doctek]

Sure, you can use Arduino and not need any of this. This Instructable is for those who want to learn about the AVR processors above and beyond the Arduino.

Whatever works for you.

[doctek]

Oops - I thought you were commenting on my I2C instructable. Ignore what I just said.

A better answer is: See the Follow-Up to this Instructable that uses the IR sensor for feedback and a PC to enter and track the temperature.
http://www.instructables.com/id/Closing-the-Loop-on-Surface-Mount-Soldering/

Basically, it uses Arduino for the microcontroller portion.

[dausmus]

Comment on Lead Free issues-

I use lead solder due to the tin whisker problem. I will switch to lead-free once that is really solved, otherwise anything I build without lead-based solder that needs long-term reliability will require inspection on a regular basis, especially if the board is under thermal or mechanical stress. References:

http://en.wikipedia.org/wiki/Tin_whiskers

http://www.jedec.org/news/pressreleases/jedec-and-ipc-release-tin-whisker-acceptance-testing-standard-and-mitigation-prac

http://nepp.nasa.gov/WHISKER/ (This one has some great pictures of whiskers!)

http://www.usatoday.com/tech/news/2007-10-05-tin-whiskers_N.htm

I think metallurgists will eventually solve the problem, but for now I am sticking with lead-based solders and non-ROHS components unless I have to use lead-free for other reasons. And, my volume is very low.

[aterhune1984]

Is it possible to substitute the MOC3042M-ND for the MOC3043M-ND?  the electrical properties seem to be slightly different... The only reason i would want to is the 42 is on backorder.  Any thoughts or is it safer to wait for the 42 to come in?

[doctek]

The 3043 will substitute perfectly. The only difference I can see is that the current required to trigger the part is only 5ma instead of 10ma for the 3042. It'll work just fine. Cost is about 10 cents higher for the 3043 - I don't expect that's a show-stopper.

[snarfer]

Great instructable! I like to get my stencils from Ohararp.com, supposedly kapton can be laser cut slightly more accurately than mylar. Also, might want to note that when you need to replace those QFN parts you're going to need some sort of rework station with a hot air gun, unless you want to put the entire board back on the hot plate... I got a nice one for 90 dollars, but there were cheaper ones available for half that. Finally, just wonder if you've had any trouble with uneven heating on your hot plate, any ideas on how to address that. I was soldering one board recently that was kind of long and thin, the ends never melted but the middle overheated. Maybe I will try putting a piece of aluminum underneath next time.

[doctek]

Great input! Thanks for adding to the discussion. Think I'll try Ohararp.com next stencil. Sounds like a good product at a fair price. Pololu has been very good however. Yes, for rework you need a hot air gun. In fact, some places (like curiousinventor aka Scott Driscoll) suggest two of them, one front and one back. I mostly have avoided rework by using cheap parts and small designs. I know I'll have to deal with this someday. The plate I have seems to give even heat, but I've used only small designs. If you're trying a new design for the first time, then plan to get at least one or two extra boards to experiment with. The aluminum plate idea is one I've seen mentioned in other places. I think it's a good one, but you'll have to work out the correct solder profile if you use one on your hot plate. Again, thanks for the input.

[doctek]

I no longer recommend Pololu - there prices are through the roof! See my post above about this.

[SMTstencils]

hello there,

Since you your discussion involves SMT stencils, I thought you may want to know that now Applied Electronics provide both Mylar and Kapton SMT stencils. Starting this coming Thursday (June 25th), we will take order for Kapton too. You can choose either Mylar or Kapton of size 8.5x11 inch (actual size for Kapton is 8.5x12) without any restriction on the area or number of components.

Mylar comes in thickness of 3 and 4 mil.

Kapton comes in thickness of 3 mil only.

Visit the following site for detail

http://www.applied-electronics.com

Regards
Applied Electronics

[doctek]

This looks like a good option. Kapton has been recommended to me for this application since it is tougher than mylar and can have cleaner edges. Besides applied electronics, I have also heard good things about
www.ohararp.com/Stencils.html

I no longer recommend Pololu. Not only do they use mylar, their prices have gone sky high!

[adelsmud]

Hi, there I was wondering where it would be ok to apply small 'blobs' of solder paste on the pads instead of getting a custom stencil made up. Also will a pcb without solder mask work? Thanks and great tutorial by the way.

[doctek]

Applying small blobs is certainly possible. The solder paste often comes in syringe tubes, and needles are available just for what you suggest. Another way to use such a tool is to apply a thin line of paste across several pads. In fact, some people have reported success simply using an Xacto knife to apply a thin layer of paste to the pads (without trying to separate each pad's paste from the ones next to it). Once the paste is on, the part is placed on it, heat is applied, and capillary action / surface tension does the rest. So this is certainly possible. Be careful not to smear the part around as you place it!

Trying this procedure on a board without solder mask is more risky, but might work. I can't say it won't; only that the risk goes up with each move away from using a stencil and solder mask. The risk also increases as pad spacing decreases. So the tinier the part, the more important solder mask and stencil become.

[jensenium]

You're a good man, and thorough.

[cgo]

Hi again. I completed your tutorial, it works great ! However, I'd like to point out this part: Sharp S116S02. It can replace the MOC+the SSRs+diodes+1W resistors, all in one $5 part. It needs a compatible heatsink, too. Regards, Charles.

[doctek]

Thanks for the suggestion. Looks like a real nice part! Here's two thoughts: 1 - Since it's a triac, it needs a snubber circuit as discussed in the Sharp apps note. One of the reasons I chose SCRs was to avoid needing a snubber. Not a big deal, just a couple of extra parts. 2 - The heat sink tab is not isolated, so you'll need an insulating washer on the external heat sink. Watch out for that. The Digikey part number is 425-2399-5-ND and the price is $6.05.

[musick7]

Hi Doc! This is Awesome! I would like to make my own and I was wondering if you had a YouTube Video of this in ACTION? I would love to see it working. Not that I don't believe you after all seems many have made one from your Tutorial in all the Reply's. I guess what I'm getting at is, How difficult is this to make? I am very confident in my Soldering Skills and would love to add this to my tools. Is this expensive to make? I have quit a few different MicroChips, Resistors, Caps and so on... If I had to purchase all the Parts needed and I did Bargain shopping online getting the best price and giving each distributor a Max. of 10 days to receive parts ordered from them. What would be the total Cost? And if you don't have a YouTube video where is another great demo of something just like this one? Does anyone else have there Plate on YouTube that they made from this Tutorial?

[ewertz]

Can you safely pass on the snubber if you're sure that your load will only be resistive (like, for this?) ? I'm asking because I've already got a small handful of triacs around... Great write-up. Thanks!

[doctek]

IF your load is purely resistive (like this hot plate), then you can skip the snubber. This fact is discussed later in the project for the SCR circuitry used.

[tundrawolf]

This is fantastic! I used to work with used test equipment and related equipment and wave soldering always fascinated me. I would like to see more steps and WAY more pictures. It is a lot for me to digest. But it is great nonetheless.

[jimk3038]

Hey, Checkout this Non-Contact Laser Thermometer at Harbor Freight - it's only $30 bucks and it has a range up to 520 degrees C. For $30 bucks you can't go wrong. Plus, I wonder if it could be hacked so that it could be connect to a PC. Mine is already on order! Jim

[Polymorph]

It is important to know that most noncontact pyrometers are calibrated for 80% reflective objects. IE, grey to black. Just so you know. Lighter objects read as a lower temperature than they actually are. A dark green PCB may read correctly or a little low. Tshirt pressers often stick a piece of black high temp tape (looks like masking tape) onto a white or natural aluminum press when testing the temperature.

[ewertz]

Grrrrr. It's $60 now.

[doctek]

Great find! Perfect if for this application. Thanks for the pointer!

[doctek]

Have a look at my update to this Instructable here: www.instructables.com/id/Closing-the-Loop-on-Surface-Mount-Soldering/

In it, I show how to use an IR sensor from Harbor Freight to build closed loop control for this process.

[jimk3038]

 Very nice.

Someone else has also reverse engineered the Cen-Tech 96451 handheld unit I bought. Here is a link to that discussion too:

www.instructables.com/community/Cen-Tech-Thermometer-to-PC-Hacking

Here is a link to the Cen-Tech thermometer I used:
www.harborfreight.com/non-contact-laser-thermometer-96451.html

The model I used has more range (up to 500C) and the spot size is 8:1 instead of 6:1. But, my model cost 1/3 more at $30 bucks. Sounds like either model will work.

Thanks for all the info,
Jim

[countable]

I do surface mount as part of my course, and I have to say it's very interesting. We have a machine that links to a PC and will automagically pick and place the components for you (assuming the solder paste is already there). For a laugh, i made a TDA2822D mini amplifier by placing all the components by hand. The only thing i can say about it is it helps to use a microscope at that kind of level!

[Gilius]

Instead o using solder paste can I apply solder to the board instead? (like tinning) The solder would then re-melt and secure the components to the board.

[doctek]

Yes, I suppose you can do that. If you do, be sure to put additional flux on the board before your reflow it (Scott Driscoll discuss this on his site which is mentioned in the Instructable).

Now here's why I'm not keen on this approach.
- Getting solder on all those tiny pads is a lot of work. The whole point of using a stencil and solder paste is to avoid that work.
- Making the solder reasonably level is nearly impossible, so some pads may not start off touching. This may or may not be a problem, but if there's too much solder on a central pad, then the resulting pillow could keep the other pads from ever touching. Paste and the correct footprints avoids this problem.
- It's really easy to lift a pad from the board with a little too much heat when you're tinning them. Tinning the part itself is a better plan, but I still don't like it.

Hope that helps.

[Chris2048]

What about chemical tinning? I imagine it wouldn't have these problems.

[doctek]

Not sure exactly what you mean here. Do you mean a process such as hot air leveling (HAL)? Generally, this isn't used for fine pitch surface mount due to poor planarity. Also, you still have to put on solder paste. Perhaps there is another process of which I'm not aware (very possible!). Could you elaborate on what you have in mind?

[Gilius]

It's an alternative becuase I can't seem to get my hands on any paste. :(

[doctek]

Paste is something you must order. Check out the link in the Instructable. You can just buy a syringe of it.I suggest getting lead-free paste for a few reasons: - It's lead-free! (no fume dangers) - It's less sensitive to a little bit of temperature during shipping so you don't have to go a real expensive route (with dry ice, etc.) - If you use the hot plate method in the Instructable, you've got plenty of heat available. - It's the wave of the future - lead will be gone completely in a few years. So get lead-free, no-wash solder. Works great. If you live in Denver, you may be able to will-call it at a supplier, but most anywhere else, you'll have to order it. Nobody seems to stock it.

[dagenius]

The fumes are actually from the flux boiling off of the solder, but in general I try to avoid lead, for various reasons, one of which is that if I touch it, THEN it may get into my system.

[Gilius]

I'm not from the US. I'm in Romania and while we have large electronics suppliers here they refuse to sell to hobbyists because we're not companies. :( However I prefer working with lead solder because it has lower melt temperature and it's less brittle than the ROHS stuff. Considering it's paste it's not a problem but I'll never normally use lead-free solder wire. I hate the stuff.