Surface Mount

Jay Kickliter

I've decided to go to surface-mount for my future PCBs, mainly to keep sizes down. My question is, what size resistors to use? Most of time time I use 1/4 or 1/8 watt through-hole resistors, because they are small. But these are usually just pull-up or protection resistors, and dissipate almost no heat. What size surface mount resistors do you guys recommend in these positions? It looks like they get so small that handling would be very difficult.

Has anyone tried the skillet reflow technique that Sparfunk recommends? Also, what about stencils? I use Advanced circuits student deal for my PCB ($33), but they don't seem have deals on stencils. Do you make your own stencil?

Franklin

Your answers will depend on how many boards you plan to make of one design. For one offs I just use solder paste from a syringe and a small bore needle to lay down small dots on the pads and the skillet to heat it up. Recently I got a hot air rework station but have not built anything since I got it so I can't comment on how useful it would be for assembly. It works great for salvaging parts though.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
- Stephen

uxorious

The EIA Standard package size for SMT resistors will guide you in your wattage ratings.

0201 1/20
0402 1/16
0603 1/10
0805 1/8
1206 1/4
1210 1/3
2010 3/4
2512 1

If you want a quick changeover from TH to SMT, start with either 1206 or 0805 packages. 1206 is widely used in SMT DIY kits because they are easy to solder by hand.

I use NIC components quite often, and here is selection guide for standard thick film resistors: http://www.niccomp.com/catalog/nrc.pdf

Depending on the type of resistor you use, you can get higher power ratings in a smaller package, but they will cost more. I have seen a 1 watt in a 1206 package.

The availability can also guide your design. A lot of designs I see are using 0805/0603 unless current or board real estate is an issue. You can get all standard values of 0805/0603 pretty much anywhere (digikey, mouser, etc).

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
~~ dRu ~~

Phil Pilgrim (PhiPi)

Jay,

At your age you probably still have a steady hand and good eyesight. So I don't think you'd have a problem with 0805s or even 0603s. 0402s are probably way too small for even the most intrepid hand solderer, though. Stencils are pretty pricey for one-off stuff. DigiKey sells solder paste in syringes. I use Sn63/Pb37 with an R500 flux, but that comes in only the large (10 cc) size. They carry smaller syringes with no-clean flux, but I don't have any experience with those. Be sure to keep the paste in the fridge, or it'll go bad really fast. I bought a box of plastic tips that screw onto the syringe's Luer fitting. They're color-coded by size, and the ones I use are blue (0.016" ID). You'll go through a lot of these, but they're cheap. Get a good pair of non-magnetic tweezers, too, for placing the parts onto the pasted pads. I use an IR toaster oven to melt the solder, so I can't advise on the skillet method.

-Phil

Jay Kickliter

Thanks for tips. I'll order a medly of 805 resistors. When laying down the paste for something liken a QFP Propeller, do you have to coat the whole length of the pad, or just put a small blob? Or should I attempt using the QFN?

Phil Pilgrim (PhiPi)

Don't use the QFN! What I do with the QFP is lay a thin bead of solder paste lengthwise along each side (perpendicular to the pads), where the leads will touch down. The solder's surface tension will suck it onto the pads when it melts. There will inevitably be a few solder bridges, which a good solder wick can easily mop up.

-Phil

Jay Kickliter

"Don't use the QFN!"

Too bad, one of the reasons I wanted to try SM was because most of the gyros and accelerometers out there are QFP, and I didn't want keep using Sparkfun breakout boards. Is QFN impossible without a machine?

Gadgetman

The problem with QFN is that the pads are hidden UNDER the IC.
This makes it impossible to remove solder bridges without desoldering the entire IC.

QFP, though, has no such problems.
(Thank you, Parallax, for releasing a QFP version of the Propeller. )

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Don't visit my new website...

Leon

I've soldered a few QFN parts without any problems using drag-soldering. The trick is to extend the pads slightly. The technique works very well with QFP chips, as well.

Leon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Phil Pilgrim (PhiPi)

There's also that thermal pad underneath that needs to be dealt with. BTW, Leon is a 10th Dan blackbelt (with oak leaf clusters) soldering wizard. My experiences with QFNs have been mostly unpleasant.

Jay, reading and rereading your post about Sparkfun breakouts, etc., I'm at a loss parsing it for a reason to avoid QFP. Can you elaborate?

-Phil

Jay Kickliter

I meant that they are QFN.

Erik Friesen

I have just got done with a small batch of boards using a pololu plastic stencil. The stencil takes about 70% of the work out of surface mount. These plastic stencils cost about $30 and for a few boards you can't beat it, although I am sure that a stainless stencil would be much nicer. One thing I think would work too is to build a small stencil with just an 0805 hole and move it around the board. Without a stencil for soldering the prop, the smallest bead you can lay will probably be about right.

If your board is big enough to use 0805, that is about the best size. 0603's are quite doable also. 0403's are doable but without a stencil you will find it difficult.

I don't know why, but that skillet method seems like a bad idea to me.

Last project I used the no-clean pb-free paste from alldav. It seemed to work well, although not like good old lead. The water soluble lead free from alldav I would not recommend. I could not get consistent reflow and the joints looked bad.

Phil Pilgrim (PhiPi)

The problem I see with the skillet method is the temperature gradient through the thickness of the board. To get an adequate melt temp on the top surface, the bottom surface may have to be hot enough to scorch or delaminate the board, or pop the through-hole connections apart. I'm not speaking from experience, though, since I've never tried this method. But infrared toaster ovens are fairly inexpensive. I use one without a convection fan, but I wish it had one, especially for larger boards.

-Phil

Leon

Another problem with a skillet is that the temperature might not be very even. However, SparkFun used to use one quite successfully when they first started, although they don't use one now. Not having a skillet I experimented once with a stove hotplate, and it looked as though the technique should work. A toaster oven should be a lot better, though.

Leon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 2/6/2009 5:53:13 PM GMT

Philldapill

Jay, it seems you and I have been trying similiar things, in regards to some of your other posts. I've just started SMD stuff with resistors/capacitors in the 805 package. 805 is a little hard to handle since they are so tiny, but 1206 is a good bit easier. If you have the board space, and the chips on hand, go with 1206 until you get really good.

Also, I've just used my oven. I set it to about 350F, which is just hot enough to flow the solder, but not too hot to kill most chips. I put it on a pizza pan that has holes in it. Works great. The only problem, is sometimes the resistors tend to "tombstone" where the surface tension of the solder makes them stand up on end. I get less than 1 out of 10 do this, which is an easy fix by hand.

Jay Kickliter

I'll tell you what guys, I'll give the hot plate a try and report back with my results.

Phil, you say an IR oven is not that expensive. Is that "relatively inexpensive"? I looked all over, and everything I saw was a bit pricy. But I don't know what to look for.

rjo_

Walmart has a pizza maker that might work... an open air, rotary design...can heat top or bottom or both... it is about $30.

And if it doesn't work, you can still make a quality pizza for about $3.

Leon

Most toaster ovens are IR, with quartz heaters. I paid £20 (GBP) for a small one from Argos.

Leon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Phil Pilgrim (PhiPi)

Jay,

I paid about $100 for mine. It's metal (not plastic) and has a digital temp control and timer. It lacks a convection fan, though, which — from experience — I now consider essential for boards larger than a few square inches. Here are the features to look for:

1. Infrared (quartz) elements.
2. Accurate temperature setting, be it analog or digital.
3. Convection fan.
4. A built-in timer is handy for timing temperature profiles.

In reading the reviews for an oven, if you see something like "browns toast quickly and evenly", it will bear further investigation. Check out Target and Amazon.com. You should be able to find something that fits your budget.

VERY IMPORTANT: Once you've used the oven for soldering boards, don't reuse it for food! The same applies to a skillet.

-Phil

Bob Lawrence (VE1RLL)

I've found the features in this toaster oven interesting. Apparently it heats quickly as well. Also listed below are a few features that may make it good for a SMT oven.

If you select enlarge image you can see LED's for the temp gauge starting at 180 degrees,250,320,320,400,470 (hard to tell if I have the temps as shown because it's a little hard to read from the picture.)

Sanyo Digital Convection Toaster Oven:

*Twin convection outlets heat food evenly
* Digital convection oven with twin inlets
* 1,350 watts
* Full-width top and bottom quartz heaters
* 40-minute digital dial timer with lighted countdown indicators
* Five temperature control settings
* Easy-to-use digital controls
* Stainless steel finish
* LED's to show the oven temperature

Picture and a complete features list:
www.walmart.com/catalog/product.do?product_id=3875024

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Aka: CosmicBob

Phil Pilgrim (PhiPi)

That looks like a nice oven for the price. The limited temperature selections are a concern, though. 400F (204C) may not be hot enough for final reflow, and 470F (243C) could scorch the boards. (220C is the maximum recommended for tantalum caps before they suffer damage.) In ignorance, I used to use 460F (238C) and often scorched the boards and browned the yellow tantalums.

-Phil

kelvin james

Check out the doc on microwave reflow soldering. I don't think it is a reality for the hobby type, but lots of interesting info anyhoo for reflow tech.

**

Bob Lawrence (VE1RLL)

One exceptional feature of this model is the convection fan, which operates continuously while the oven is preheating and turns on and off to maintain a constant temperature. While on, the fan is amazingly quiet. Two inlets distribute the heated air throughout the oven chamber for faster, more even cooking. The heat is generated by two oversize quartz heating elements -- one on the top and one on the bottom. The elements glow bright red within seconds of activation and cycle on and off during cooking. The combination of heaters and fan provide fast, efficient heating within the oven chamber.

products.howstuffworks.com/sanyo-digital-convection-toaster-oven-sk-vf7s-review.htm

From the review:

Pros: Digital touchpad controls, convection fan for even heat distribution, stainless-steel housing, 40-minute timer

Cons: Limited to five temperature settings, can't reset timer until unit shuts off

Phil said...
The limited temperature selections are a concern, though

Good catch Phil. I didn't notice that at first. I considered them as indicators not limits. But if everything else met the required spec the temp settings could be modified.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Aka: CosmicBob

rjo_

Phil,

How about cooking up a programmable IR diode with enough wattage to make a small spot flow?

I can't do it, but I'm guessing it would take you about an hour[noparse]:)[/noparse]

Rich

Timothy D. Swieter

There is such things as point soldering with lasers. I am waiting for someone to try it with one of their desktop laser machines. I am not sure of the parameters of what is all involved but I saw a detailed book on the math and science of soldering and one or two chapters was dedicated to point soldering using various devices and I believe one was a laser. The book was a few years old.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Timothy D. Swieter, E.I.
www.brilldea.com - Prop Blade, LED Painter, RGB LEDs, uOLED-IOC, eProto for SunSPOT, BitScope
www.tdswieter.com

grasshopper

You fellas ever heard of a second-hand store or a yard sell? I got my toaster oven for less than $10 at a second hand store and its all metal.

Just some input [noparse]:)[/noparse]

Tracy Allen

There is such a thing as a recommended temperature profile for surface mount soldering, one that preheats to ~150 deg. Celsius for a couple of minutes, and then ramps relatively quickly and evenly up to the reflow temperature and back down. I don't know how important that is in reality, but it is often recommended for totally consistent results. Here is a nice article about retrofitting a toaster oven for precise control.
www.circuitcellar.com/library/print/0704/Lacoste_168/
It BASICally or could be PROPerly bypasses the toaster's own sensing and control logic and runs the power through an external SSR.

Here is a pre-built retro-controller:
www.pcbunlimited.com/product_info.php?products_id=78

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com

Phil Pilgrim (PhiPi)

I was in Costco yesterday and saw a nice infrared toaster oven with convection fan, continuously-variable temperature control, timer, and metal housing for $50. I can't remember the brand name, though.

-Phil

Timothy D. Swieter

Phil -

That sounds like a good deal. I paid more for mine here in Hong Kong with similar features. There was another thread where I mentioned that I built my own controller using the Propeller. I just used the oven again over the weekend and it works great. The only kink I have in my system is the thermocouple or the thermocouple software. I hope I can figure this out in the near future when I get some spare time.

Using a toaster over without a controller is possible too, just be careful to not overcook the boards - I did that once when I took my eye off the oven and thermocouple "failed" send the temp too high for too long.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Timothy D. Swieter, E.I.
www.brilldea.com - Prop Blade, LED Painter, RGB LEDs, uOLED-IOC, eProto for SunSPOT, BitScope
www.tdswieter.com

william chan

Phil Pilgrim (PhiPi) said...

Be sure to keep the paste in the fridge, or it'll go bad really fast. I bought a box of plastic tips that screw onto the syringe's Luer fitting.

Phill,

How do you tell if a solder paste has gone bad?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
www.fd.com.my
www.mercedes.com.my

Phil Pilgrim (PhiPi)

The little solder balls separate from the liquid flux and turn into concrete. When that happens, you can't force it from the syringe. (Cold temperatures increase the flux viscosity and help to keep the solder sphericules in suspension.) I'm seldom able to use an entire 10cc syringe before it goes bad, though.

-Phil