This Instructable shows how to create a Simple Solder Paste Stencil, Solder Masking with Acrylic Lacquer Spray paint and 2 sided board Vias. Along the way I will show the easiest way to add components. All without drilling holes in the PCB board. Using both surface mount and conventional components. I have been making my own boards for over 25 years, during this time I figured out most of the easiest methods. ("It is best to read through the entire instructions, before attempting these Steps")
(Note: "For Non Surface mount boards, I will show how to create an easy Solder Mask, holes must be drill for components at this stage.) If You do not want to read this entire Instructable go to "Applying the Solder Mask"
Step 1: Creating a Solder Paste Stencil With a Drill Bit.
Note("For Non Surface Mount PCBs using conventional holes and no surface mount component, Or for Users adding paste with a syringe if you do not want to create the solder stencil go to Applying the Solder Mask step.")
Lets Create a Solder Paste Stencil. Using either a laser printer or inkjet create an image on plain paper of all the solder pads and any area where you are going to add solder. Using the Image you create for your PCB will work, or edited it showing the pads only. ("Make sure is not mirrored unless necessary") Tape and glue ("Not Elmer's it is to wet.") the image squarely to a sheet of aluminum 26 to 28 gauge, an inch or so larger than the PCB board. Mount the aluminum sheet to a board for stability and as a straight edge guide for holes in a straight line. Drill (13/64" or No.54 bit, 0.055 inch) holes through the paper image into an aluminum sheet at each point where solder will be placed, (Even if it is not a surface mount component. "Not for through holes" This will also be your Solder Mast Stencil. ) Solder paste is very forgiving and will gravitate toward the copper trace as long as some of the paste is on the land. An attempt to be as accurate as possible will help keep solder from moving to an improper place. When all holes are drilled remove the Aluminum Stencil from the board. Using a flat file, file the Aluminum holes ("no rough edges") completely flat. After it is flat run the drill bit through each hole again, file if not flat, repeat as needed. ("While I used aluminum for the stencil it is also possible to use other materials") Aluminum of this gauge is available at Hobby Lobby or online. On this first attempt using this paint. I used too thick a sheet of aluminum causing excessive solder around components. 0.025 inch did not work well. This was from Menards, Lowes has the same thing. "I had used sheet metal before" Resistors and ceramic capacitor are 1206 SMD.
The finished board above has no holes except for mounting and battery holder alignment pins. Even the Copper Trace Text is fully visible through the paint. The DS3231 I'm still waiting for it.
Step 2: Apply Solder Paste to Your Board, Also Thinning Dry Solder Paste
Mount the PCB board with the Stencil on top of it securely to a board. In this case I used the board mounting hole and inserted a screw through both the stencil and PCB. On the opposite edges a used a cable hold down clamp with a screw into the wood. Align the holes with the solder pads visually ensuring most if not all the pads are under a hole. The Stencil and PCB need to be in very close surface contact so when spreading the solder paste with a Credit card or similar device the paste only fills the hole and does not squish past the hole. Enough pressure needs to be applied with the credit card, so that the Solder Paste comes in good contact with the PCB and fills the hole to the top. Exerting finger pressure to the Stencil near the current point of the application will ensure good contact between the PCB and Stencil. After filling all the holes remove the main screw holding the Stencil down. Then carefully remove the other screws without moving the stencil. Then carefully push down on one edge of the Stencil using the edge of the PCB as a fulcrum point. Then with the other end in the air carefully pull up on it to bring it off of the PCB. What you should have left is little dots of solder paste. If the paste is too thin or thick it may stick in the holes of the Stencil. If too much has stay in the Stencil it may be best to start over removing the solder from the PCB and reusing, after changing it's consistency. Experiment as necessary, thinning solder paste can be done with 90% or greater isopropyl alcohol or in the case of rosin based solder paste use turpentine, water based water. Add very small amounts at any one time, Stirring until smooth and consistent, over thinning will require sitting it out to air dry. ("When using Surface Mount IC's with many small pins, only drill 1 hole for each 2 pins, then spread the paste evenly across all pins")
Step 3: Appling the Solder Mask.
If applying Solder Paste do before this Step!
This step is very simple. Using Tamiya Acrylic Lacquer spray paint, PS-44 Translucent Green. Spray a single coat of this paint enough to cover the board with a shinny finish, do not over spray just enough to cover board. As with any paint this should be done in a well ventilated area, read cautions on the side of the can. This paint is stated for use on model cars and it is fuel resistant. Move the painted board to an area with a temperature above 50 degrees fahrenheit room temperature is best, if the temperature is above 85 degrees place in a shaded area. Depending on the temperature it will dry between one and five minutes. Let it sit for a total of 10 minutes to air out fumes. That is the Solder Mask. ("For people with conventional boards using holes, ("Nail It") get some small headed nails that will cover the lands to be soldered,suspend the PCB so it is not sitting directly on a surface, Insert a nail into each hole, spray paint as above. "If so desired it maybe heated to harden with a Hot Plate as I used in Reflow Soldering, an Oven or Carefully with a Heat Gun") I have read that some people have concerns about using Acrylic Lacquer. I have used enamel before with no problems. ("See Pictures Above") Lacquer was used with screen printed PCB mask long before other coating came along. Acrylic Lacquer is used in some cases by PCB manufactures today, but most have been replaced with epoxies. While it is difficult to solder through this paint it is easily removed with a scraping action, I used a flat bladed jewelers screwdriver. The paint at this stage is not cured yet. The paint to some extent will keep the solder paste moist but not for too long. Now is the time to install components. ("Why did I choose this brand of paint, because it is translucent and matches the color of most green PCB's"). It was hard to find this color. It is available on Ebay and online. In red, blue and green. Tamiya PS-44 Translucent Green ("I forgot to take a picture after painting the board so I just photo shopped it green.")
Step 4: Installing Components.
I am sure most of you are used to installing components, their is only one step extra, after installing all the SMD components take a flat blade screwdriver with a 1/4 inch wide blade. With the flat side of the blade push down on the components so they will be seated close the the board. ("The Next Step Soldering") TIPS Below
Component Installation tips::: TIP: Have a picture of the component layout and a component list when installing components. TIP: A locking 7 day and night pill box works perfectly for holding tiny surface mount components. ("just be sure to get a locking box or they will pop open and you know the rest") Ezy Dose 7-Day AM/PM Adult Lock Pill Box 2XL, has a bar lock similar to a tool box, not letting any compartment open without pushing a button on the side. TIP: A 5 times magnified lighted lens available on Ebay. TIP: A piece of white paper under the board while installing components, drop one and it can be found. TIP: A vacuum pickup tool made from a inkjet refill syringe or a solder paste tube. Using a modified aquarium air pump to create vacuum picks the component right out of the Pill Box and onto the Solder Paste. So much faster than a tweezers or any other method. Placing a finger over a hole in the syringe controls air flow. TIP: Using Dip sockets, bend the bottom leads of the socket at the half way point of its length 90 degrees outward. ("Why not just straight out, When doing reflow the plastic melts if in direct contact with the board. ("Hot Plate Soldering") ("A reflow oven would probably melt the socket and would require a socket intended for high temperatures.") TIP: Using standard resistors or capacitors. I cut one end to about 1/8 inch long, solder it then cut the other end to the required length and solder it. ("No holes very time consuming if doing more that one board.") Transistor cut the leads about 3/8 inch long and bend the lead ends to 90 degrees somewhat less than 1/8 inch long. Install NON Surface mount resistors and transistors with a standard soldering iron, after reflow of the Surface Mount components. TIP: Create a Stencil holder ("See Picture") I soldered together some sheet metal for a frame. Mounted it to a board and put alignment pins on the board.Hinge the rear hold down part.Install some hold downs on the edges. On future frames I will have to use small machine screws to hold the stencil to the frame, as I was using sheet metal for earlier stencils ("Two problems with steel, bits break all the time and if you start a hole and decide to move it a fraction of an inch Aluminum will allow you to do it, steel will keep going back to the original started hole.")
Step 5: Reflow Soldering With Hot Plate
Of course most of you using reflow probably are aware of Hot Plate Soldering. This is what and how I use it. The hotplate came from WalMart the lowest cost one they had, ($10.00) it has spiral heating coils. Of course it would not work well with just the spirals. Originally I use a heavy gauge table saw blade on top of the spirals, it worked well for a while, until it became so warped after repeated usages. I purchased a 6 x 6 x 0.25 inch aluminum plate. ("Ebay") It covers all of the coils and is perfectly flat. Only used one time so far, worked well! I set temperature dial on the Hot Plate to less than 4. After plugging it in, it takes about 2 minutes for the solder paste to begin smoking. Shortly their after, the solder begins to melt. When I see that the solder on the entire board is melting I unplug it. ("Keeping it plugged in burns the board.I remove PCB while hot, when it looks completely soldered.") ("As with paint the solder paste emit fumes while melting, its best not to do in the house") I have never used an oven but feel that since it is heating top and bottom the components may get hotter. The lower the temp that is used to melt the solder is best. While the solder is melting, the paint is baking to a harder finish.
Step 6: Adding Through Hole Components As SMD
After Soldering the surface mount components with the hotplate, I added through hole components used as surface mount. Axial Resistor and Capacitors: I clip the lead on one end to the desired length usually around 1/8 of an inch long. I then tinning the shortened lead and solder it to the board. Next I cut off the other end to the length needed to fit properly, tin and solder to PCB. The DIP Socket was added before the reflow, ("The pins were bent about half way down their length 90 degrees outward, avoiding direct contact of the plastic body to the board, otherwise it melts the bottom and sometimes pins inside the socket.") as of yet I have not been able to bend them evenly. Before reflow I place the socket on the paste and solder 2 opposite ends to the board. After reflow I take my soldering iron and press down on each pin firmly into the solder, in most cases no additional solder is needed. Transistors: I cut the leads to about 3/8 of and inch, then bent about an 1/8 inch at the end of the lead, 90 degrees to one side. Solder one outer pin first and work my way across, repeat soldering as necessary to seat the pins to the surface of the PCB. Header Pins: For headers to be upright I use right angle pins with the 90 degrees bend below the plastic. Since I included solder paste on the pins earlier it makes soldering these, in particular easier .("Scrap off any paint from these pads so solder flows under pins" Use a Jewelers Screwdriver") I created pads about 0.22 inches in length and about 0.060 inches wide. The leads on most right angle headers are longer that this so I shorten them using a steel scissors ("Picture Above") one of the blades I place against the plastic housing and use as a spacer, then start cutting the length of the header. I used break apart header, Cut to the number of pins needed. I place the header in it approximate location and solder one end to the board, Then go to the next pin in line and solder it on placing pressure from the iron to seat it to the board. I then add as needed add conventional solder to firmly attach it to the PCB. The next step. ("Removing the flux")
Step 7: Removing the Solder Flux
Depending on which type of solder paste you use, determines what type of Flux removal you use. ("As for No Clean Solder Flux") [ "I have seen from my experience as an Electronic Tech many boards, after they are several years old, the flux begins to turn White, creating in some cases shorts and looks very bad. Looks really bad having a White Residue all around the pins of components."] Since the paint has been baked it is very resistant to chemical removal. The Flux itself is no harder to remove than it was without the paint. I tried 3 chemicals: Acetone ("Finger Nail Polish Remover, My preferred is Naphtha Enamel Paint Thinner, and what has always worked the best was Lacquer Thinner but it removes the strips off of resistor and such. With effort it would begin to remove the paint") I scrub the board with a paint brush dipped into the Thinner. After I use the thinner I wash the board with dish washing liquid and water. Then dry the PCB with a Hair Dryer. I have used commercial flux removers and they work well but in general they are expensive. For water based flux Just Plain Water and perhaps brushing as well.
Step 8: Surface Mount Jumpers, Mini Secondary Boards and 2 Sided PCB Vias
One of the items I've used is circuit jumpers, either 0 (Zero) ohm resistors or metal jumpers as on the board in the picture. (See Photo) These shinny SMD strips of metal are available in different lengths and widths and Zero ohm resistor look like resistor. This Allows jumping over other circuit traces that normally require 2 sided boards. I mostly use them for power traces. I take laying out the circuit traces on the board like doing a cross word puzzle or trace the line through the maze. I actually enjoy the challenge of using as few jumpers as possible. Sometime 2 sided board are absolutely necessary in the case of a Parallel or Serial Port connector. Putting the connector on the edge of the board 1/16 thick. To avoid the extra cost of a 2 sided board. I make mini boards that cover only the area I need. (See Photo) By using 2 x (1/32) inch thick boards putting them together makes 1/16 inch which is the spacing between the rows of pins. To join the 2 boards together I align the top and bottom boards, (copper up on top, copper down on lower. ) drill the holes in the top board. I drill two hole in the bottom board and tie the two board together with Escutcheon Pins then drill the other bottom holes while attached to top board. (See Photo) The same applies to regular 2 side boards Insert Escutcheon Pins into the holes on top, Solder them, after all pins are soldered on top, (See Photo) Turn over board, solder the pins to the lands, putting the top of the pin against something while soldering the bottom, keeps it from falling out. Cut to proper length. Hopefully you have learned something from this Instructable. Bob B