Installing Dual Gate MOSFETS
The following tutorial explains how to work with the new crop of high performance Dual Gate MOSFET's that come in the Surface Mount Package. These marvelous semi-conductors are amazing and formerly were through hole devices that had all but disappeared from the scene. Philips (NXP) has an extensive line of these devices and they are inexpensive ranging down to as low as 17 Cents for the reverse model and 40 Cents for the standard. The terms standard and reverse refer to the arrangement of the pin outs.
We now have the devices and if one is not into designing special PC Boards how in the world do you take advantage of this "recycled" technology? In the future an article will appear in publication that describes a Dual Gate MOSFET 40M Direct Conversion Receiver. In a Part II a companion transmitter is unveiled. The heart of these two projects are Dual Gate MOSFETs. Thus the challenge is how to use these devices without creating special PC Boards. Shown below is the receiver.
Three methods are presented in this tutorial and all have been successfully employed on my project. Common tools such as a small steel ruler, an "exacto" type knife, a 0.5MM "Pentel' type mechanical pencil with HB lead, a fine point "Sharpie" felt tipped pen, and a small roll of 3M type paint masking tape are all the tools required to prepare the boards. When it comes time to install the surface mount devices you will need a headband magnifier, a grounded temperature controlled soldering iron with a fine tip, a roll of silver bearing solder (Radio Shack P/N 64-013), a small metal type cookie sheet (10 X 15 inches) and a sheet of white typing paper. Oh yes and a pair of fine point tweezers.
The Three Methods
Actually one method involves the use of a 4 pin machined pin transistor socket that can be installed on either a piece of single sided copper vector board or on a piece of single sided copper PC Board where small square islands are carved into the board. The second and third methods involve carving out mounting pads on either the vector board or PC Board. There are advantages and disadvantages to all of the methods and in certain cases one particular approach is the best.
In the case where the single sided copper vector board is used, there is virtually no board carving involved but it does require aluminum pillars to elevate the board as all wiring is done on the insulated side of the board. This approach may present some problems where the application is for a VFO or Varactor Tuned Oscillator (VTO) as the issue of mechanical rigidity cannot be treated lightly. The Single Sided Copper Vector Board is somewhat expensive as I recently paid $25 for a sheet 4.5 inches X 17 inches. But that is good for quite a few projects. PC Board on the other hand can be purchased inexpensively from a source such as eBay. The PC Board where square islands are cut into the board is a highly desirable approach where a solid mechanical mounting is needed such as for our VFO and VTO example.
Method #1, Four Pin Socket
Shown above is the 4 pin socket which is available from Mouser at around $1.50. Notice the socket has a protrusion on the right hand side and this is to provide indexing and a reference point. In our final installation reading clockwise is the Drain pin at about 1:00 O'Clock followed by the Source Pin at 4:00 O'Clock. When you look at the top view of the SMD device one of the pin outs is "fatter" than the others and this is the Source Pin. Proceeding to about the 7:00 O'Clock position is Gate #1 and at about 11:00 O'Clock is Gate #2.
No matter whether the single sided copper vector board or PC Board is used the SMD device still needs to be installed in the socket. Here is how to do it. Step #1 is to anchor the board so it will not move during the next several critical steps.
In the case of anchoring the vector board I pre-drill the corners of the board to accommodate the pillars, I also using a small drill bit I bought in an assortment of small bits to enlarge the four holes where the socket will fit on the board. For the Single Sided Copper Vector Board, first the holes to accommodate the socket are not lined up parallel and perpendicular such as you see in the photo. To fit on the board the index bump points south east. Just keep that in mind and use the index bump as a guide. However before inserting the socket into the board in addition to making the holes slightly larger to fit the socket, I also take a 1/8" drill bit with a 1/8 Knob fitted to the shank of the bit. A few twists of the drill bit on the top of the hole and this clears the copper so there are no shorts from the pins to ground. You can take a small drop of super glue and glue the socket to the board. Then place the vector board with its pillars on the white sheet of paper which sits inside the cookie sheet. Temporarily tape the elevated board to the paper sheet using the masking tape.
In the case of the flat copper board, in this version the 4 pin socket was first soldered to 4 squares that had been carved out of a piece of PC Board. That carving process is described in method three. The advantage here is the socket while elevated from the board is solidly attached. Subsequent connections to the device are made to the same copper squares where the 4 pins are attached. Next tape the board to the white paper as above
I take four pieces of buss wire about an inch long. The cutoff ends of through hole resistors or capacitors works very well for this application. I insert the wires into each socket hole insuring that the wire "bottoms out". Next I bend over each wire so that it form a a right angle and clip each lead to about 1/16 of an inch. Then it is time to remove the DUal Gate MOSFET from the anti-static packaging. I wear cheap thin type rubber gloves while doing this step. Carefully empty the contents of the package on the white sheet so you can see the device against the white background. Using a pair of fine point tweezers, pick up and place the DGM on the top of the socket insuring it is right side up and not upside down. You will know the proper way as when the device is right side up all pins are "flat" to the surface. Next insure that the "FAT" pin, the Source is in the lower right hand quadrant at approximately the 4:00 O'Clock position. Now carefully slide the DGM under the four pieces of wire and align it so the wire is touching each pin. This may take a bit of work but this method does provide a better anchoring during the soldering process.
The soldering process involves setting the iron temperature about mid-range (400 to 500F) and placing the soldering iron on the wire to heat the area and let the solder flow onto the pin. This is not WELDING! Look carefully at the photograph and you will see there is a small soldered area. Use the headband maginifier. To aid in keeping things steady I use the Pentel pencil to "pin down the DGM" during the soldering process. If you use care and do not solder the wire to the socket --if you mess up you can simply pull the device from the socket. In the solder to the Board methods, removing the device usually involves a total destruction of the DGM.
Method # 2 Connect the Dots
This method uses the single sided copper vector board with pads being cut on the board to accommodate the SMD devices. The desirability of this method is that both SMD and through hole components can be utilized and perhaps aside from the vector board socket method enables fast prototyping. The process involves making a rough estimate of where the devices should go on the Board. My approach is to take the schematic and fit all of the parts to the board using the through hole components as a means to set the real estate required and then to identify the amount of space needed for the SMD pads. For every pad I lay out the pads so they will have at least one hole in the middle of the pad. This is so I can make connections on the underside of the board and run at least one lead up through the board and solder it to the pad. Typically the pads are large enough that components near-by can simply be soldered to the pad. The center hole is extremely useful when bringing a power lead to the device.
The process for creating the pads once the real estate location has been determined is to roughly sketch out the pad area by connecting the dots with the felt tip pen. Then the trick is to cut the copper material outside of the dots so you are left with a net pad --not something less. I start by scribing two lines using the steel ruler and the exacto type knife. Taping the board down to the cookie sheet will aid in preventing chasing the board all over the work area. Once the lines are drawn then carefully remove the material between the two lines. The photo below documents that process. Take your time and use care when handling the exacto knife. I would strongly recommend doing one or two practice boards so you get the hang of it and then you won't have to make ay excuses about it being "Ugly Type" Construction. There are only two critical cuts in this process when making the pads for the Dual gate MOSFETS and those are made the ones that separate the pads into four quadrants. About 1/32 of an inch is an ideal cut as it allows physical separation so there are no shorts yet provides plenty of solder area to connect the pins of the device.
The Soldering process for the DGM's entails placing the board on the white paper (in the cookie tray) and taping the board down. For the soldering iron use the same heat setting as in Method #1. Next remove the device from the packaging and move it to the pad area but not on the pads at this time. Next place a dab of solder on the pad where the Drain goes, the upper right quadrant and immediately wipe it off with solder braid. Next move the DGM into position and this is where the Pentel pen comes into play. Once I have the DGM lined up, I hold my breath and with the tip of the pencil gently push down on the center of the DGM and hold it in place so it does not move. With my right hand I heat the area near the Drain and the solder residue melts and temporarily holds that pin in place. Slight corrections to the alignment can be made at this time but be careful you don't break the fragile solder tack on the Drain. Next proceed to the opposite corner and heat the area in front of Gate 1. After a bit, flow some solder on the board and move forward to the pin. That is it. Next do Gate 2 and then the Source and then a final solder of the Drain. There are tutorials on the internet how to do this and all it takes is a little practice. The process: Heat the Board, Sneak Up with solder toward the pin and don't oversolder the connection. Don't spend a lot of "dwell" time with a hot soldering iron on thses somewhat sensitive devices. That is it you are done.
The soldering process
Method #2 Adding Hardware to the Board
Method #3, Cutting the Squares
Cutting the squares on a piece of PC Board is very similar to that of the single sided copper vector board only there are no dots to connect. The process for laying out the cut areas and the removal of material is identical. Even the soldering process is the same. This method is ideal for uses such as VFO's and VTO's as everything is solidly mounted to this board. Layout is a little more critical but if an area with many squares is fabricated then it is a matter of where do things conveniently go. This process will require far more board cuts and is labor inetnsive but the material is a lot less costly than the single sided copper vector board. If one really gets skill with the layout process then the number of squares to be cut can be minimized -- that comes with practice. In this particular example I taped it to my workbench and this was about a 5 minute job from start to finish. Starting by drawing the squares on the copper using the Pentel mechanical pencil, then lay down the two 1/32 inch lines and remove the material in between the lines. The only critical juncture is the meeting of the four corners where the device is placed. That is it. As a suggestion take a piece of scrap material and practice using this tutorial as a guide. In practice say you have a board about 3 inches by 3 inches, one method would be to drill the corner holes first and then using a 12 inch X 12 inch piece of plywood, screw down the cicruit board to the plywood and this also can be left in place when it comes time to install the components.
Soldering Surface Mount Components
There are some tools I use that are what I consider mandatory to work with Surface Mount Devices (SMD). Here is my list of items
Headband Magnifier (This is an absolute must!)
Start first with a work area that has plenty of light and arrange the lighting so it does not cast shadows on the working space, which in this case is the cookie sheet with a white paper liner. The next step is to tape the board down to the white paper. It is assumed that no matter which methodology for construction that the DGM’s are installed first. Thus the board would be flat and can be easily taped down. The reason for the white paper is that the components are very small and typically are a dark color so they easily show up against a white background. Once the component are removed from the package care must be exercised to not touch any of the leads, as they are static sensitive devices. This is where the Mechanical Pencil really comes in handy. I use the pencil to push and align the DGM on to the board and insure the proper alignment onto the pads. The Source is the “fat” pin of the four pins and for the standard model is in the lower right hand quadrant. [For reference purposes the standard DGM pin out is shown on Figure 2.]It is important to have your soldering iron hot enough to melt the solder so it flows but not so hot that you ruin the device. My iron goes from about 200 to 800 F. I find something mid-range is perfect –so about 400 F. I start by placing a small dab of solder in one of the corners typically the Drain, (upper right hand quadrant) and then immediately wipe off the excess with solder braid. Next I align the device using my mechanical pencil and when I have the 4 corners aligned, using my left hand I gently press down on the top of the device with the pencil. The HB lead is soft and so has some give to it. I hold my breath while doing this step. Then using my fine point soldering iron I “sneak up” on the Drain by placing my iron on the board near the Drain. So as the solder start to melts under the Drain pin, the device is tacked down enough to solder the other pins. Check for alignment and insure you have not moved the DGM from its centered location. Assuming all is well “sneak up” to the other connections by heating the copper area near the pin to be soldered and then apply solder as you move the iron toward the pin. Since the Drain was tacked down, next in the sequence is Gate 1 (opposite quadrant) and then finish up with Gate 2 and the Source. Now go back and properly solder the Drain. Now is the time for a lecture about soldering versus welding – no welding desired or required! An 80-Watt Radio Shack soldering iron is way too big for the fine work that is required! See figure 12 for a view of a soldered DGM. If there is one clear message, practice doing this before you are ready to make a formal board.
See below for a tool built by my friend John King W5IDA, It is made from standard plumbbing type harware and is an excellent hold down tool to keep those pesky SMD's from flying about. Its made out of plumbing parts, mostly small copper pipe and a few fittings. Size of each leg is about 2" - 3". The business end is made of #12 house wiring. Pressure placed on the component can be varied by varying the length of the wire with the screw on the top leg. Less wire makes the top piece more horizontal and increases pressure.A few $$$ and a little time will save you much grief.
The process will go smoother and your final product will look a lot better.73'sPete N6QW