In spite of their convenience, the cost of solder paste stencils has prevented many hobbyists from using them. But the use of polyimide film has brought the prices within reach... if you know how and where to get them.
Surface mount devices (SMDs) are rapidly replacing through-hole components in commercial electronic manufacturing. As a result, more and more components are available only in surface-mount packages. For a while, hobbyists hand soldered SMDs, but now many have put together their own solder reflow ovens similar to the one in this article. With the ability to reflow solder, they embraced the use of SMDs and began to look for faster ways to use them. One obvious problem to be solved was the application of paste solder prior to placing components on the PCB; the answer was obvious: solder paste stencils.
Solder paste stencils allow you to quickly and easily deposit solder paste on your custom PCB (printed circuit board) with a swipe or three from a plastic card. With a little practice, it takes only a few minutes of setup time and less than a minute per PCB to apply the paste.
The longest lasting stencils are made from metal (often stainless steel) and are good for thousands of operations. If you are going to build that many boards, metal is the best choice. However, most hobbyists only make a few boards of each of their designs, and the cost of buying a metal stencil is often out of reach. Some make their own stencils from aluminum cans and achieve good results, but that process involves chemical etching which is not for everyone.
Fortunately, there is another alternative: a material called polyimide (marketed by DuPont as Kapton) can be laser cut to produce solder paste stencils that are good for at least a couple of dozen boards, and the prices are surprisingly low. One good supplier of polyimide solder paste stencils is OSH Stencils. Prices for their stencils start at about $5, and are based on the size of the stencil. Production is quick, service is good, and shipping is fast. The stencils used for this article are from ALLPCB.
Designing a Solder Paste Stencil
Most of the work of designing a solder paste stencil is done in the PCB layout software that you use. All of the better programs such as Eagle, Kicad, and DipTrace include this feature as part of Gerber file creation. Eagle calls it the "cream" layer; Kicad and DipTrace (the program favored by this author) call it the paste layer.
Whatever the paste layer is called, it is based on the size, shape, and position of the pads of the various surface-mount components in the design layout. Thus, a stencil can be no more accurate than the patterns (footprints) of the parts in the design.
The exact procedure for generating a Gerber file for the paste layer depends upon the software you are using. However, the general process is the same in every program; the drawing is from DipTrace, and is the top layer from a PCB layout that will be used as the example in this section.
As you can see, there are some surface-mount components and some through-hole components, but the solder paste stencil is only used for the SMDs. The screen (also from DipTrace) is the final step in creating a paste layer for the example design. Notice that the heading on the window shows that Gerber Files are being exported. The Top Paste layer is highlighted in blue, and the box labeled "Paste Mask for SMT Pads Only" is checked.
The next drawing is the stencil paste layer that was created based on the PCB layout and the settings shown in the preceding window. Note that the yellow rectangles represent the holes that will be in the stencil, and that there is one hole for every SMD pad in the top layer of the PCB. Also note that there are no holes in the stencil for any of the through-hole components.
It is very important to realize that the holes in the stencil are slightly smaller than the SMD pads; that's a good thing since the solder paste will be spread out slightly when each component is placed.
Exactly how much smaller the holes in the stencil are than the pads is controlled by the entry in the "Paste Mask Shrink" box; That is .003 in, which means that all four sides of each pad will be reduced by .003". Thus, for a pad that is .087 inches wide and .098 inches high (like the four large pads on the left center of the PCB,) the hole in the stencil will be (.087 minus .006 equals) .081" wide, and (.098 minus .006) equals .092" high. The holes for all the pads on the top layer will likewise be .006" narrower and .006" shorter than the corresponding pad. It is important that you understand that before moving on.
Consider the patterns (footprints) for some common SMD types. You should recognize the pattern for J1 as being used on the top layer of the PCB in the example. It has nine pads associated with it: the four large mounting pads plus five smaller pads for each lead used in a USB connection. As previously discussed, the large pads are .087" wide and .098" high, and the holes in the solder paste stencil are .081" wide and .092" high. That seems perfectly acceptable...but what about the smaller pads on J1?
The five lead pads on J1 are each .098" wide and .020" high. Deducting .006" from both of those dimensions gives a stencil hole size of .091" wide and .014" high. The width is fine while the height seems a bit small, but still will probably be okay. And using that same logic indicates that all the other stencil holes should be okay except perhaps those SSOP-16 lead pads at the right center of the stencil.
The SSOP-16 lead pads are .013" wide and .064" high; subtracting .006" from .013" leaves only .007" for the stencil hole. That will not allow enough paste through the holes to produce good solder joints. Fortunately, DipTrace provides a solution. The amount of paste mask shrinkage can be set for all components on a PCB as previously shown. In addition, the paste mask shrink can be set independently for each individual component on the PCB. In the case of the SSOP-16, experimentation has proven that reducing the shrinkage from .003" on each side to .002" produces good results. This leaves the holes in the stencil for the SSOP-16 pads at .009" wide, which will provide for solid solder joints.
Of course, the chances that you are using DipTrace and the same exact footprints with the same pad dimensions as used in the preceding discussions are quite small. In addition, the dimensions stated are not be taken as absolutes, but rather as illustrative examples. You must determine the paste mask shrinkage that works for your patterns. As a general guideline, paste mask holes should be at least 70% of any dimension of the the pads that they are meant for. If you apply that rule to the SSOP-16 pads discussed, that would mean that the paste mask hole for the .013" wide pads should be at least .0091" which is essentially what was chosen in the preceding paragraph.
One final note about solder paste stencil dimensions: the thickness of the stencil also has a bearing on the size of the holes. Thicker stencils hold more solder paste than thinner stencils in holes of identical width and height. An increase from .003" to .005" in stencil thickness produces a 69% increase in paste volume on rectangular pads of the same size. Thicker stencils are also more durable.
Using a Solder Paste Stencil
This section shows how to use a solder paste stencil to apply paste, how to place a multi-lead component, and how the part should look after reflow and cleaning is complete.
The first step in using a solder stencil is to secure the PCB on a work surface in such a way that the PCB does not move in any direction. The PCB is held in place by two L-shaped pieces of plastic that are affixed to the work surface with blue painter's tape. Alternatively, you might use any stiff material that is the same thickness as your PCB. Note how the PCB is restrained from movement up/down and side-to-side, yet the entire front surface of the board is exposed.
Next, align the stencil with the PCB so that each hole in the stencil is centered over its corresponding pad on the PCB. Secure the stencil with masking tape. As you note, this PCB has only one SMD (surface mount device,) a TQFP-44 IC in the lower center section of the board. This makes aligning the stencil easier, but the principle is the same regardless of the number of components to be reflow soldered.
The TQFP-44 pads under the stencil. Notice how the holes in the stencil are smaller than the pads, and are centered over the pads. This is important to ensure that the solder paste is deposited in exactly the right place. The blob at the top of the picture is the solder paste that is to be spread on the pads through the holes in the stencil. There is much more solder than will be needed for this part, but the excess will be wiped away with the spreader card and used for the next PCB. Sharp-eyed observers will also notice that each of the holes in the stencil is outlined in white; that is leftover paste from an earlier spread attempt that failed.
A plastic card with rounded corners does a good job of spreading the paste without damaging the stencil; the card has no embossed lettering which makes cleaning the card easier. An ordinary credit card can be used as well.
Spreading the solder paste so the holes in the stencil are correctly filled takes a little practice, but is easy to learn. The objective is to have every hole in the stencil evenly filled with paste. The white spots on the stencil are small specks of solder paste. They do no harm and will be cleaned away before the stencil is stored.
After the stencil is carefully lifted from the PCB surface, the remaining solder paste can be inspected. Every pad has a small deposit of solder paste, and there is no excess between or around the pads.
A prior paste application effort that was not so good. Note how there is paste connecting some of the pads together. However, the chances are good that the reflow would have been successful due to the wicking action that draws the molten solder onto the pads and the feet of the IC. At worst, some minor cleanup with solder braid would have solved any problems.
Using whatever method works best for you, place the component on the pads in the proper orientation. The IC is not properly placed; the legs at the bottom are barely touching the pads. Gently nudge the component in the proper direction to the point where all the legs are mostly over the proper pads. For example, the legs on the right side of the IC are not perfectly positioned, but they are close enough for good reflow.
It is likely that some of the solder paste will be smeared when the component is nudged into place, but most likely the reflow will still be successful because the molten solder will migrate to the pads and can actually float the part into position. Again, each of the legs must be mostly over the correct pad.
A successful reflow from less-than-perfect solder paste application and component positioning. You can see some solder and flux residue between the pads, but there are no solder bridges, and all the feet are soldered to the correct pad.
The part has been cleaned with 91% isopropyl alcohol and gentle action with a soft bristle toothbrush. As you can see, the solder and flux residue is gone, the part is well soldered, and can be expected to function with no further intervention.