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REMOTE CONTROL BY BOB YOUNG Working with surface mount components – easier than you think The introductory article on the new radio control system last month has really put the cat amongst the pigeons. Readers have reacted with horror & revulsion at the thought of using surface mount components so it seems appropriate to discuss these devices in detail before we go any further. I know, I know. I promised that this month would see the circuit of the receiver for this R/C series but the best laid plans of mice and men and all that . . . I have found it neces­sary to make a few revisions to the design which has now been test flown over quite a few weekends and before I go into print with these, I want some more test flying done. So to stop the editor from taking a fit of apoplexy, I olution is no longer so quiet, for the movement towards surface mount is now rolling along at full steam ahead. Everyone, it appears, is now climbing on the SMD bandwagon. So much so that the usual ridic­ulous 8-12 week component delivery times have blown out to the insane. Surface mount devices for model work, and model aircraft in particular, offer many advantages over “I can now hand-assemble a surface mount PC board in a fraction of the time it took me to complete a comparable through-hole assembly. And, as an added bonus, the finished result is a much better unit all round”. am bringing forward this article which would otherwise have been presented in a couple of months’ time. One of the revolutions that has quietly overtaken the elec­ t ronics industry over the past few years has been the introduc­tion of surface mount PC board assemblies. This quiet rev72  Silicon Chip conventional leaded (through-hole) components. To begin with, the components are very small and thus quite light. More important, however, is the fact that the components sit flat on the PC board and thus in a crash will not bend or shear off at the lead/body junction. Thus, the final assembly is much more robust and gives fewer failures in service. For these reasons, I have chosen the surface mount format for the R/C project to be presented in the coming months. Therefore, I feel that it is appropriate to present a de­tailed article on how to handle surface mount components by hand, in anticipation of the requests I will receive for this sort of information. Most people I have spoken to over the past few years regard surface mount with some apprehension, if not active dislike. This is a pity actually for as I will demonstrate, surface mount offers many advantages, even from a hand assembly point of view. Obviously the major thrust towards surface mount comes from the fact that it was originally designed for rapid machine assembly, with the cost savings that accrue from automation. But even from a hand assembly point of view, surface mount has much to offer and I have now arrived at a point where I would sooner work with surface mount components than through-hole. When working with surface mount, you just simply solder each end of the component to the PC pads and that is it; no spending hours bending and shaping leads. There is no fumbling around trying to push leads through holes or working in a forest of leads, or trying to get solder and iron into an almost impos­sible to reach location. Nor is there any going back and snipping leads which fly around the room, turning the workshop into a safety goggle area. When I look back at that form of assembly, I shudder these days. I can now hand-assemble a surface mount sembled in this manner in the future. Thus, it is not too difficult to envisage a situation in the not too distant future where some components will only be available in surface mount form. For these reasons, it is important that people involved in electronics start to come to grips with the techniques involved in building and repairing in surface mount. What is surface mount? Fig.1: this is a typical surface mount assembly machine. Note the large reels which take a minimum of 3000 components. PC board in a fraction of the time it took me to complete a comparable through-hole assembly. And, as an added bonus, the finished result is a much better unit all round. Disadvantages Possibly the major disadvantage is the fact that some components are not marked and this forces upon people working in surface mount an almost fanatical sense of discipline with regard to storage of components and recording of components placed. Service is likewise a little more difficult when dealing with unmarked components. For our prototype and service work, we use a tray drilled to accept pill bottles. Both the lid and the bottle are marked and we even punch holes in the lids so that they do not have to be removed from the bottle. Components are removed from the bottle with tweezers. The worst situation is one such as I encountered with the receiver front end developed for this current project. Here I was working with up to six prototypes simultaneously, all using a large number of unmarked components (mainly capacitors). Every receiver had to be numbered and every component change had to be recorded, because once you put that unit down, five minutes later it was impossible to remember what was on that board. The real problem arose when I got tired and forget to record a change. What was on the board? There was no way of knowing except by removing the component and measuring the ca­ pacitance or replacing it with a known value. This is all very tiresome and consumes a lot of time. Capacitors can be purchased with markings but are more expensive and thus few and far between. Once the unit is in production, then document control becomes extremely important from a service point of view. These disadvantages are minor though when compared with the benefits obtained from this very important development in component technology. From the electronic enthusiast’s point of view, the import­ant point is that surface mount is here to stay and more and more equipment will be as- Surface mount is a system of assembly utilising components which are designed to be placed by machine flat onto the surface of a PC board which has already been prepared with a screened solder paste. This PC board and its components are then run through an oven or wave solder machine to complete the soldering process. There are many forms of soldering techniques and it is not proposed to delve too deeply into this aspect of surface mount. Our concern today is with the hand assembly of these components. However, it is important for servicemen to know that one very important difference between wave soldering and infrared reflow, for example, is that infrared reflow does not require the compon­ ent to be stuck to the PC board with epoxy adhesive before it is soldered. This makes servicing just that much easier than dealing with components stuck down with epoxy. The reason for the epoxy being needed for wave soldered SMDs is that the solder wave will wash the components off the PC board unless they are stuck down first. It is interesting to note that we regularly assemble doubled-sided surface mount boards without epoxy, using infrared reflow. In this case, the solder paste is sufficiently tacky to hold the components in place on the underside of the PC board while it moves through the reflow oven. The automated PC board assembly process begins with the preparation of a PC board with specially shaped pads. These are designed to ensure that the resultant soldering process results in a satisfactory bond without skewing the component around. The shape of the pad and the way the tracks meet the pad are very important because we are dealing with very light compon­ents in a fluid (molten solder) with very high surface tension. This is doubly important when using infrared January 1995  73 reflow without epoxy. Always keep in mind here that we are dealing with compon­ents sitting flat against the PC board with no lead through a hole to stop movement. Poorly designed PC board layouts can result in components skewing around and shorting out on the component next to them. As our usual component spacing on tight PC boards is 0.020-inch, even a small movement can have serious consequences. Square pads with heavy tracks coming in at 45 degrees are notorious for pulling components around. Some PC board manufacturers appear to have difficulty understanding this, for I have often rejected PC boards from manufacturers when they have substituted square pads for our rounded rectangles. Component assembly The physical assembly process begins with the screening of the solder paste onto the PC board. The solder paste mask is etched into brass shim and the thickness of the shim determines how much solder will be applied to the join, the usual thickness being 0.008”. The solder paste mask is 0.005” less in diameter than the pad size, to stop solder from spilling over the edge of the pads. The screened PC board is then run through a surface mount assembly line. There are a myriad of these machines and Fig.1 shows a typical example. Note the reels holding the components. Ordering components for surface mount is not for the faint heart­ ed as the minimum reel size number of vias in the PC board. I am very reluctant to put a track under an 0805 using rounded rectan­gle pads. However, the Japanese do it regularly and appear to have no problems. Other components we will encounter in this project are the SOT23 transistor and diode packs, a little 3-prong package and the SO14 and SO16 surface mount IC packages. Kit presentation Fig.2: this shows the receiver board with all components soldered at one end. Each component is soldered at one end first while being moved into position with a pair of fine tweezers. Note that the board is shown here slightly larger than actual size. run through the soldering station. The finished product is a joy to behold, with bright solder joins which are fully wetted and with good meniscuses. That is provided nothing goes wrong, which it very easily can do if the entire process is not monitored very carefully from PC board design to soldering. The basic building blocks in the surface mount component range are of course the resistors and capacitors. These are usually little rectangular blocks and are often defined by their size in inches. Thus, a 1206 resistor, the most commonly used size in our component range, measures 0.120” x 0.060”. There are many sizes in the range, however in this project we will only encount­er a few of them. Fig.2 shows some of the more common components in surface mount. The small “Hand assembly of a surface mount PC board is very easy provided you have a good magnifying lamp or glasses, a soldering iron with a very fine tip, a pair of tweezers, a solder sucker & a steady hand”. holds 3000 components. Resistors come in reels with a minimum of 5000 per reel. Surface mount does not lend itself very well to small runs, as the setup costs are high. On long runs, it is the only way to go. Once the PC board is loaded with components, it is then 74  Silicon Chip capacitors in the receiver, for exam­ple, are 0805 (0.08” x 0.05”). The Japanese manufacturers have virtually standardised on 0805 and most Japanese equipment uses this size component. I find 1206 more practical as two tracks can be slipped under a 1206 safely, thus reducing the The kit will be presented in two forms: (1) an assembled surface mount PC board leaving the through-hole assembly to the customer; and (2) a full kit which will feature a bare PC board and packs of surface mount components. A note here on component marking. The resistors are always clearly marked in the usual 3-digit format (4-digit for close tolerance components). Thus, a 100kΩ resistor will carry the marking 104 or 1003 for close tolerance. The capacitors may or may not be marked (most commonly not). If they are marked it will be in a 2-letter code (usually) which will look something like AO. The manufacturer’s code list is necessary if you wish to decode this symbol. Likewise, the markings on the SOT23 packages are in a 2-digit format and again the manufacturer’s list is needed to decode it. For example, a BAS16 diode carries the marking A6 and the BFT25 transistor, V1. So be very careful when handling those unmarked components. You have been warned! Hand assembly of the surface mount PC board is very easy provided you have the correct tool kit. Do not under­take hand assembly unless you have the following equipment: a good magnifying lamp or glasses, a soldering iron with a very fine tip, a pair of tweezers with points less than 0.020” thick, a solder sucker and a very steady hand. To begin, lay the bare PC board down flat on the table with the surface mount pads facing upwards. You will notice that the pads are already tinned but they have been levelled so that there is not enough solder to hold a component in place. Thus, the first step is to pre-tin the pads. Align the PC board so that the diode and transistor packs (SOT23) present the single pad to your soldering hand. Tin one pad only on each component and this pad should Fig.3: the essential components you need for hand assembly of a surface mount board are magnifying spectacles (or a magnifying lamp), a fine tipped soldering iron & tweezers with very fine tips. be the pad closest to your soldering hand. Tin the single pad only on the SOT23 packages and only one pad on any IC packag­es. Now using tweezers, pick up the first component and slide it into place near the appropriate pre-tinned pad. Apply the tip of the soldering iron to the pre-tinned pad and slide the compon­ent into the wet solder, stopping at the correct location on the pad. This is usually with the component centred on the pad. It is very important to pick up the component from a flat surface so that the points of the tweezers do not protrude below the bottom of the device. This can otherwise result in a component sitting above the PC board and thus prone to cracking. This is also the reason that we solder one leg only. If both pads were tinned, the component would not sit flat against the PC board. Let the solder cool before removing the tweezers. You now have your first surface mount component on the board. That wasn’t too painful was it? Continue placing components until all of the surface mount components have been mounted. By this stage you will have all the components in place but with only one pad soldered on each de­vice. Now align the PC board so that the unsoldered pads are facing your soldering hand and apply the tip of the iron and the solder simultaneously to the pad. Watch the heat, as these com­ponents are quite delicate. A quick dab will do it. At this point you may like to touch up the original join if you feel it needs it. At some stage in the assembly, particularly for the receiv­ er PC board, you will have had to place two components side by side which are only 0.02” apart (PC board programs such a Protel Autotrax work best in inches, hence the Imperial measurements). Care should be exercised to ensure that these components are parallel to avoid shorts. You may also have to file the points of your tweezers to get them in between the components. File the points so that they are of equal length and less than 0.02” thick. In case you are wondering how thick 0.02” is, don’t worry. You will soon find out when you try to assemble the receiver. If you cannot place R8 and C11 for example, because your tweezers won’t fit, then they must be thicker than 0.02”. Probably the most difficult component to solder is the SOIC package. The legs on these packages are only 0.05” apart and there are lots of them. It is very easy to get a solder bridge on these leads. If this happens, use a solder sucker and a minimum of heat to clear the bridge. Also keep the iron tip clean and make sure it stays pointy. Removing components Finally, a word or two on removing components. On a general level when servicing surface mount equipment, it must be kept in mind at all times that some components may be fixed with epoxy as well as with the solder. This makes removal more difficult, howev- er epoxy will let go at elevated temperatures and on small components the epoxy is not usually a problem. Larger components are a different matter. In our project, epoxy will not be encountered, but you may still have some difficulty removing components unless you know the tricks. As you cannot snip one end and lift the component away from the board as with through-hole components, you must use quite a different technique. Packages such as the 1206 can be removed by quickly heating each end in turn two or three times until the component moves. Once it moves, slide the component sideways and upwards with the tip of the iron. Usually the surface tension will cause the component to adhere to the iron tip. As a result, at the end of the day you end up with a sponge tray full of components. The SOT23 package presents more of a problem. Begin by heating all three legs in succession and then hooking the single leg upwards with the tip of the iron. Once the single leg is up the other two will come away easily. Alternatively, the legs may be cut with a sharp knife and each leg removed individually with the tip of the iron. There are, of course, specially shaped sol­dering iron tips for all of these components but they are very expensive. The IC packages are by far the biggest headache. A sharp knife is the next best to a dedicated iron tip. Run the knife along the legs close to the IC body. These legs are quite thin and will cut through easily. Care should be taken to ensure that too much downwards pressure does not translate into sideways force which will lift the tracks. Likewise, when tracks are warm do not lift upwards with too much force or the bond between the tracks and the fibreglass substrate will be broken. One big problem is replacing a device on pads that have solder on them from a previous component. It is very difficult to get the component to sit flat against the PC board unless the pads are properly cleaned first. Solder wick is a help here but my old mate Boris has the most elegant solution. He solders in stereo, with an iron in each hand. Removal and replacement is quick and easy in this manner but the board must be anchored and you SC must watch the heat. January 1995  75