You, too, can do 0402!

By Bill Shaw

0402 capacitors ready to be mounted.

 

Increasingly components are available only in surface mount packages. While surface mount assembly requires some different tools and techniques than through hole assembly, these tools are not particularly expensive and the techniques are not difficult to master. This article describes some tools and techniques available for assembling prototype or low volume surface mount printed circuit boards. Boards have been successfully built using these techniques containing parts as small as 0402 and 0.5mm pitch, but that's where I draw the line! :-)

Schematic capture and PC board layout software is available for free from two PCB vendors (at least), ExpressPCB ( http://www.expresspcb.com ), and PCB123 ( http://www.pcb123 ). Both vendors' software packages work well and will allow you to create the artwork for your board. Both vendors offer low cost prototype boards. Each vendor has advantages over the other in some area. ExpressPCB offers what they call their MiniBoard service that has to be the best deal in PCB manufacturing. It gives you three 2.5" x 3.8" PCBs for $59, delivered! Express PCB's layout software will show you all the pads in a net when you click on one, but PCB123's layout software goes one step farther by telling you if you've connected the net you're routing to the correct pad when you make the connection. PCB123 charges $50 to create the stencil file, but ExpressPCB charges $30 for the top copper layer artwork. Each PCB vendor has advantages and disadvantages; the choice is yours.

There are other options for printed circuit boards that on the surface look OK, but smell a little funny once you get a little closer. Internet facades are springing up all over that are nothing more than 2 guys with a computer selling other people's PCBs and marking them up. PCBUnlimited.com is one of these companies. They'll take your order, mark it up and then either ship it off to a real PCB manufacturer in the US or even send the work to China. Skip the middleman and save youself a few bucks. Deal directly with the manufacturer. Same goes for StencilsUnlimited.com, just 2 guys with a server in the garage marking up your SMT stencil order and shipping it off to a real company.

If you really must buy your boards from China, PCBCart has a good, simple web interface and is very inexpensive.

 

To build your surface mount board you'll need the following equipment:

 Fine point tweezers

 Fine point razor knife.

 Solder paste

 Syringe with a fine tip or a solder paste stencil

 Fine point soldering iron

 Lighted magnifying lens

 Toaster oven

 TempTell Reflow Controller or temperature indicator stick

 

It helps to get organized before starting. Some component suppliers will print the reference designators on each component's envelope for you. If your component supplier offers the feature, make sure you enter the reference designators for your components in the 'Customer Reference' field in their online order form. If not, make sure you manually write the reference designators on the packages before you start assembly. Go through all your components and make sure you have everything before you think about touching the solder paste. This is a good time to write the reference designators on the part packages if you did not take advantage of the component supplier's 'Customer Reference' field. This step will make the component placement go much quicker and less error prone.

After all of your parts are present and accounted for, prepare the work area. Place a sheet of clean white paper on the work surface. This makes it easier to find a part when you drop one. Get a couple of clean paper towels and have them nearby, you'll need them to wipe any excess solder paste from the syringe tip and to clean off any that accumulates on the tweezers. Get your assembly drawing (top layer pads and silk screen), parts list, and schematic out.

This is probably a good place to talk a bit about solder. Most solder contains lead, which is a hazardous substance. Any unused solder, empty solder containers, and solder laden rags must be treated as hazardous waste and must be disposed of in an appropriate manner. Talk to your solder supplier about the proper methods of disposal for these items. Lead is readily absorbed through the skin. Wash it off thoroughly if you get any solder on your hands. Don't eat, drink, or smoke while handling solder, and don't put your hands near your face while assembling your boards. Don't use your reflow oven to heat food after you've used it to reflow PCBs!

Solder paste has a short shelf life and normally needs to be consumed within about 6 months so you don't want to buy more than you're going to use in that time frame. A good source of solder paste is Solder and More.

 

 

 The easiest way by far to apply solder paste is with a stencil. Stencils are available from many sources, but QuickStencil sells a prototype solder paste stencil kit for $99 that contains all you'll need for a small batch of surface mount boards. The kit contains the 2 plastic board holders shown here, the prototype stencil, and the stainless steel squeegee for applying the solder paste.

Another good option if your layout is not too complex is to use a mylar stencil. The finest pitch these mylar stencils can handle is approximately 0.5 mm. These mylar stencils are very cheap, around $25 each, and are available from Pololu, a robotics supply company. Pololu actually makes the stencil, unlike the internet facades like StencilsUnlimited.com who just take your order, tack on $50 to the price, then ship it off to a real manufacturer to do the actual work.

If your project just can't afford a stencil it is still possible to place the solder paste by hand, it's just a bit more tedious. I used to do the first board using this technique, then buy a stencil after I have the first one running and have made any necessary tweaks to the design. Nowadays I'd buy a mylar stencil, and only spring for a stainless stencil if the geometries get small. If you're flush with cash it will make your life a lot easier to spring for the stencil on the first pass, even though you may have to buy another once you tweak the board to get it fully working.

The next step is to place the solder paste. If you are using a stencil, the instructions on the PCB Express web site will guide you through the process. If you don't have a stencil then read on, this section is for you!

Solder paste is available in 35 gram syringes for manual placement of the solder paste. 35 grams is enough to solder down a LOT of components. Paste sold in syringes is generally a bit 'thinner' than the paste sold in larger containers for use with stencils, allowing it to flow through the tip better.

Manually placing the solder paste and placing the components is done under the lighted magnifying lens. Good light and magnification make all the difference in the world for these steps!

Using the syringe to place solder paste on the larger pads like 16-SOT and 0805 components is pretty straightforward if a bit time consuming. Place a dot on each 0805 pad, extrude a small bead on each longer pad. Don't over-do the solder, all you need is a small dot. Too much can cause solder bridges. If you smear any paste on the tip, wipe it off with the paper towel. Keeping the tip clean will help you place the solder neatly.

 

 

 

 

For thin narrow pads like these, get a small blob of solder on the end of the syringe, dab the solder near the end of the pad to stick it, then stretch the bead by moving the tip over the pad as you extrude a little more solder paste. With a bit of practice, you'll be able to drop a thin bead of paste on the pad. Don't worry if it isn't perfect, we'll have a chance to clean it up in a bit.

 

 

 

 

 

Another technique that works well on very tight pitch parts is shown here. These pads are on 0.5 mm centers. The syringe tip is significantly larger than the pad. Place a bead of solder over the whole area and spread it out quite thin, about 0.010" thick should suffice. Next, take the razor knife and draw a clean break between each of the pads. You can feel and follow the edge of the pad with the knife blade.

 

 

 

The solder paste looked like this when finished, and reflowed without bridging. The excess paste lying around the pads pooled up and pulled together while reflowing, so it looks clean when done.

 

 

 

When all the paste is down it is time for inspection. Check the board thoroughly and make sure you got all the pads. Use the razor knife to clean up any solder paste that has overflowed onto a neighboring pad. You can also remove part of the paste on a pad with the knife if you've placed too much. Keep the tip of the knife very clean, wiping it after each pass through the solder to keep it from smearing. You don't have to scrape up every bit of excess solder, it will pool and pull together when it reflows. Keep a clean line between each of the pads.

Once you are satisfied with the paste it is time to start placing components. Start with the least fragile components and work your way through to the more fragile components. Headers and connectors first, resistors and capacitors next, then diodes and transistors, and finally the more complex silicon components.

Each package of components you pick up will have the reference designator written on it (you DID do that, didn't you?). Dump out the number of parts noted on your parts list for that item and start placing in the solder paste using the fine tip tweezers. This is where your assembly drawing comes into play. If you've bought the low cost prototype boards as shown above you will not have a silk screen on the boards to tell you where to place the parts, so you'll need to find each component on the assembly drawing and place the part in the corresponding location on the board. Frequently my designs will have different population options for different functionality from the same PCB, so I will use a highlighter to mark the components to be placed for each option in different colors on my assembly drawing. This reduces the amount of thought necessary at assembly time, and reduces the number of errors.

When placing the parts, push the component down into the solder paste a bit to make it stick. This is necessary to hold the part in place during the remainder of assembly and transport to the reflow oven, as well as to help keep the parts from tombstoning. Tombstoning is when a part, like a capacitor or resistor, stands on end during reflow. Having both ends of the part stuck in the paste helps prevent that from happening. This is a good time to final check the amount of solder on each pad as you are placing the components.

After placing a part wipe the tips of the tweezers on a clean paper towel to ensure you did not pick up any solder paste. Even a slight bit of paste will make the small components stick to the tweezers.

You may need to add a touch more solder here or remove a bit there. Placement of the parts needs to be close, but does not have to be perfect. If the pads are the correct size the parts will self-align a bit when the solder melts.

 

 

The next step is to reflow the board. There are two techniques for reflowing a PCB using a toaster oven. The first is to use a temperature indicator stick. Place a small chunk of this stick on your board. When this chunk melts the board is hot enough to melt the solder. This method should work OK in most cases, but a more precise method of reflowing is available by using a TempTell Reflower to control the toaster oven. The two graphs below show the difference in the temperature profile using these two methods.

 

 

 

 

 

 

The TempTell Reflower monitors the temperature of the PCB being reflowed and controls the temperature according to the user-defined parameters. The toaster oven is plugged into the outlet on the Reflower, and the temperature probe is placed on the PCB. The Reflower will then control the oven temperature to attain the user-specified reflow profile. The user can define the reflow profile by specifying the slope of the drying ramp, the drying temperature, the drying time, the reflow ramp slope, the reflow temperature , the reflow time and the cooling ramp or time.

The TempTell Reflower is calibrated during initial set-up for optimal performance with the specific oven being used

The TempTell Reflower can be purchased from Articulation LLC.

 

 

 

The temperature probe is placed on the PCB (brown wire touching the right hand board) so that the temperature being monitored is the temperature at the surface of the PCB..

 

 

 

 

 

This is the temperature ramp-up profile of a standard Proctor Silex toaster oven set to 450 degrees Fahrenheit. Note the 75 degree overshoot before it settles back to 450 degrees.

 

 

 

 

 

 

This is the temperature profile of the same oven controlled by a TempTell Reflower. The temperature profile starts with the drying ramp. Once it reaches the drying temperature it levels off for the drying time, then heads up the reflow ramp toward the reflow temperature. Once it reaches the reflow temperature it maintains that temperature for the reflow time, then ends the reflow cycle.

This oven does not have quite enough power to achieve the optimal reflow ramp, which makes the entire cycle a bit longer than it should be.

Note how the oven holds temperature after the end of the cycle. The door needs to be partially opened at the end of the cycle to allow the board to cool.

 

 

 

 

 

 

The next step is a close inspection. Check the board carefully for solder short and tombstones. These will be repaired with a hand soldering iron and a bit of patience. The traces for SMT components are typically quite small and it is easy to damage them with too much heat. Keep the iron set to a low temperature and keep the heating time to a minimum for any rework that is necessary. It is also possible to thermal shock the SMD components, so be careful not to touch the body of the SMT components with the soldering iron.

 

 

 You should now be ready to start debugging your new surface mount design. Feel free to email any questions and I'll do my best to answer them.

Bill Shaw
b.shaw@comcast.net
Articulation LLC
176 Dennison Road
Westbrook, CT 06498
860-399-5334

For information on PCB layout and fabrication see:

http://www.PCB123.com
http://www.ExpressPCB.com

For information on SMT solder paste stencils see:

Pololu
QuickStencil

For more information on the TempTell Reflower see:

http://www.ArticulationLLC.com

 

And if this has scared you off you could always use a brick and mortar PCB Assembly house:

http://www.Doltronics.com

Last updated: 2/26/09