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How to solder SURFACEMOUNT DEVICES Many electronics enthusiasts hesitate to build projects involving surface-mount devices (SMDs) because they’re daunted by the prospect of soldering such tiny parts to a PC board. But it can be done. Jim Rowe shows us how. . . I T’S TRUE THAT SMDs are not really intended for manual assembly. They’re designed for automated pick-and-place machines and reflow soldering ovens. The problem is that more and more ICs and other components are becoming available only in SMD form. As technology marches on, it’s becoming necessary for everyone to get used to working with SMDs. 222 1.5 3.00 '1206' CHIP RESISTOR 1.5 0.6 3.00 '1206' CHIP CAPACITOR 222 0.6 You may already be familiar with the simpler SMDs like resistors, capacitors, diodes and transistors. Some of these are shown in Fig.1. Note that they’re all shown twice actual size, for clarity. We’ve used these in various projects published in the last few years, and shown how they can be soldered onto a PC board: use a soldering iron with a fine conical or ‘flattened conical’ tip 1.3 2.0 '0805' CHIP RESISTOR 0.5 1.3 2.0 '0805' CHIP CAPACITOR 0.5 1.25 1.7 1.0 SOD-323 DIODE 2.92 4.9 1.3 1.0 SOT-23 TRANSISTOR OR DIODE Fig.1: a selection of common SMD components, shown here twice full size (if we showed them normal size they’d be hard to see in some cases!) 22  Silicon Chip and very fine (0.71mm OD) resin-cored wire solder. Figs.2 & 3 show how this is done. The basic idea is to hold the chip or device in place while you tack-solder one or two of its leads to hold it in position. This then allows you to solder all of the leads to their pads in the usual way. It needs to be done carefully and fairly quickly, so you don’t damage 6.6 3.9 1.5 8-LEAD SOIC 6.1 2.3 D-PAK POWER TRANSISTOR OR REGULATOR siliconchip.com.au either the SMD or the PC pads by overheating. You also need to make sure you don’t apply too much solder, which can cause fine solder “bridges” to short between pads or tracks. If you do get solder bridges, they can be removed by applying the end of some fine de-soldering braid to the top of the “bridge” and briefly applying the tip of your soldering iron to the top of the braid, so the end of the braid heats up to the solder’s melting point and ‘sucks up’ the excess solder by capillary action. OK, so what is the real problem with SMDs? Um, it’s the large SMDs with umpteen dozen closely spaced pins. TIP OF TOOTHPICK 'HOLD DOWN' SOLDERING IRON TIP TINY DROP OF SOLDER UPPER TIP OF CROSSOVER TWEEZER 'HOLD DOWN' SOLDERING IRON TIP SMD CHIP COMPONENT COPPER TRACK AND PAD 1 LOWER TIP OF CROSSOVER TWEEZER PC BOARD Holding SMD chip in place while applying a tiny solder drop with soldering iron tip to “tack” one end SOLDER TACK NOW HOLDING CHIP IN PLACE SOLDER 1 Holding SOT, SOD, SOIC or similar semiconductor device in place while tacking one pin SOLDER TACKED LEAD NOW HOLDING DEVICE IN PLACE SOLDER 2 Other end of SMD chip now soldered to pad in normal way 2 Pin or pons on other side of device now soldered to pads in normal way 3 First end finally re-soldered in normal way 3 Pin on first side re-soldered, others soldered in normal way Fine-pitch ICs More and more VLSI (very largescale integration) devices now come in SMD packages like that shown opposite and those in Fig.4 – quad flat packs (QFPs) with anywhere between about 44 and 208 leads. The lead pitch can be as fine as 0.4mm – less than 16% the pitch of 0.1”/2.54mm used in most familiar ‘dual in line’ IC packages. The width of the leads can also be as fine as 0.18mm (that’s right – only 180mm!), so the actual spaces between the leads can be as small as 0.22mm/220mm. Now it is possible (just!) to solder a 44-lead MQFP device with 0.8mm pitch leads like that shown in Fig.4 using a fine-tipped soldering iron and the technique shown on the right in Fig.3. That’s providing you are extremely careful, have a very steady hand and don’t mind having to use the soldering braid to remove the almost-inevitable solder bridges. If you can do this consistently, you are a champion! The real problem arises when it Fig.2: the basic steps involved in manually soldering smaller SMDs like those shown in Fig.1, using a fine-tipped soldering iron and very fine resincored wire solder. The steps for resistors and capacitors (left) are much the same as those for SOT, SOD and SOIC devices (right). UPPER TIP OF CROSSOVER TWEEZER 'HOLD DOWN' SOLDERING IRON TIP LOWER TIP OF CROSSOVER TWEEZER 1 2 Holding down MQFP or similar “Gull Wing” IC package while tack-soldering one corner lead Opposite diagonal pin of device now tack-soldered in same way, to locate all pins on their pads. FINE (0.71mm OD) RESIN CORE SOLDER 3 Close-up view of a 44-lead MQFP device with 0.8mm pitch (lead spacing), after being reflow soldered using a low cost snack oven. siliconchip.com.au UPPER TIP OF CROSSOVER TWEEZER 'HOLD DOWN' First “tacked” pins now re-soldered, others soldered in normal way. SOLDERING IRON TIP LOWER TIP OF CROSSOVER TWEEZER 1 2 Holding down PLCC or similar “J-lead” IC package while tack-soldering one corner lead Opposite diagonal pin of device now tack-soldered in same way, to locate all pins on their pads. FINE (0.71mm OD) RESIN CORE SOLDER 3 First “tacked” pins now re-soldered, others soldered in normal way. Fig.3: manual soldering of SMD ICs with lead pitches of 0.8mm or more can be done in the same way if you’re VERY careful but be prepared for the accidental creation of solder bridges between leads – and having to remove them using solder wick. As you can see there’s not much difference in approach between ‘gull wing’ and ‘J-lead’ devices. March 2008  23 LEAD PITCH 0.8mm LEAD WIDTH 0.38mm 10.0 LEAD PITCH 0.5mm 14.0 LEAD WIDTH 0.22mm 10.0 14.0 2.45 44-LEAD METRIC QUAD FLAT PACK (MQFP) ALL DIMENSIONS IN MILLIMETRES 1.60 100-LEAD LOW PROFILE QUAD FLAT PACK (LQFP100/SOT407-1) (BOTH DEVICES SHOWN 2x ACTUAL SIZE) Fig.4: the key dimensions of a 44-lead MQFP device compared with those for a 100-lead LQFP device – both shown twice actual size for clarity. You can see why the fine-pitch devices can’t be soldered in manually or even by wave soldering. comes to devices with lead pitches of 0.4mm or 0.5mm, like the 100-lead LQFP device shown in Fig.4. These packages are not even suitable for automated wave soldering, let alone manual soldering. The leads and gaps between them are just too narrow. The only way to solder these devices is by reflow soldering. This process involves applying solder paste to all of the tiny pads on the board (using a laser-cut stencil and squeegee system), then placing the SMDs accurately in position on the board. The boards are then placed on a conveyor belt and passed through an ‘IR reflow oven’ at a controlled rate, using infrared radiant heating. Inside the oven they move through areas with temperatures set for preheating, followed by a ‘ramp up’ to above the melting point of solder and then a ‘ramp down’ to well below the melting point. This is known as a ‘reflow soldering profile’. Using this approach, SMDs with a lead pitch of 0.4mm can be soldered to boards safely and with a high degree of reliability, at the same time as all of the other SMD components. The main drawback is that a commercial IR reflow oven is very expensive (many thousands of dollars) and thus beyond the reach of enthusiasts and even many small manufacturers. Getting laser-cut solder paste stencils made from your PC board CAD file is not cheap either. 24  Silicon Chip So the challenge is to find a much cheaper way of soldering these finepitch SMDs into PC boards. Luckily, there is a way! amount of solder paste to every pad on the PC board where an SMD lead or contact area is to be soldered. This is shown in the upper two diagrams of Fig.5. This technique simply it isn’t practical for small manufacturers or enthusiasts. A much simpler approach involves applying a thin ‘stripe’ of paste along the pads for the SMD leads, as shown in the lowest diagram in Fig.5. The stripe of paste is only a millimetre or so wide and can be applied using a fine brush, a very narrow roller applicator or a fine spatula with a 1mm wide tip. You’d think that applying a continuous stripe of solder paste in this way would be ‘asking for trouble’ for it to SOLDER PASTE SQUEEGEE STENCIL About solder paste Solder paste is available from the better electronics stores. It consists of tiny spheres (<50mm in diameter) of tin-lead solder (63% tin, 37% lead), suspended in a water-soluble paste or gel of flux. It’s typically sold in fairly large plastic syringes, holding about 80 grams of solder paste. This is actually far too much for the average enthusiast, because the ingredients in the flux apparently have a shelf life of only six months after manufacture, even when stored in a refrigerator. Yet 80g of paste is enough to solder many hundreds – even thousands – of SMDs. So while solder paste is available, it’s a pity that it isn’t sold in much smaller quantities – say 5g or 10g. This would mean a lot less wastage. By the way, when you buy solder paste, make sure you store it in a refrigerator so you’ll at least maintain its six-month working life. And if you store it in a fridge that is also used to store food (of course!), place the syringe in an air-tight container because both the solder spheres and the flux apparently give off toxic fumes. Applying the paste As mentioned earlier, largescale manufacturers use lasercut stencils and a squeegee system to apply just the right COPPER PADS PC BOARD APPLYING SOLDER PASTE USING A STENCIL AND SQUEEGEE SOLDER PASTE LEFT ON PADS AFTER STENCIL IS REMOVED THIN SOLDER PASTE STRIPE OVER PADS LOW COST ALTERNATIVE: MANUAL APPLICATION OF SOLDER PASTE 'STRIPE' Fig.5: for reflow soldering, largescale manufacturers apply solder paste to the board pads using a squeegee and a very thin stencil, laser cut from the PC board CAD file (top). This leaves the paste neatly on the pads (centre) but this is not feasible for enthusiasts. Luckily for fine-pitch SMDs, a very thin paste stripe (bottom) is almost as good. siliconchip.com.au Above: a closeup view showing a thin ‘stripe’ of solder paste applied manually to the pads for one side of a 100-lead LQFP device, with the tiny solder spheres just visible. This stripe is a tad uneven in thickness – a little too thick near the left end, and a little too thin towards the centre. Below: closeup of the same board after the device had been reflow soldered using a snack oven. Despite the 0.5mm lead pitch, there were no solder bridges. soldering process - not easy to repair! So the most important thing about this manual approach to applying the solder paste is to take your time and care in making the stripe as even in width as you can. It’s easiest to do this with the board under a magnifier lamp or even a low-power stereo microscope with illumination. I’ve also found that a very thin and narrow-tipped (about 1mm) spatula seems to make it easier to apply and even-up the paste stripe, although a very narrow ‘applicator wheel’ (I made one myself) was almost as good, and easier than a fine brush. Whatever you use, the main ingredient is time and patience – applying solder paste is a bit like trying to spread microscopic caviar evenly on a sheet of glass. In fact, since you have plenty of paste, do a few dry runs on a sheet of PCB copper laminate. Placing the SMDs form bridges between pads, during the reflow soldering process. However the secret of this approach is to make the paste stripe very THIN – only about 100mm wide or two solder spheres thick. If it’s no thicker than this, the result is that surface tension and capillary action causes the solder spheres to ‘pull themselves together’ into the gaps between each SMD lead and its board pad, when they melt during the reflow soldering. Most of the solder spheres in the paste between the pads get sucked into the molten solder directly under each SMD lead, leaving very few to form bridges. Not too thick, not too thin If you make the paste stripe too siliconchip.com.au thick, there WILL be enough spheres left in the gaps between pads to form bridges. On the other hand, if you make the stripe too thin, there will be insufficient spheres to pull together and form a good bond between each SMD lead and its pad underneath. So erring in this direction results in ‘missing joints’ after the reflow Once the solder paste has been applied to the board, you can place your fine-pitch SMD(s) in position, with their leads over the board pads ready for the reflow soldering process. Large-scale manufacturers use a pick and place machine to place all of the components on the board in one pass – not just the fine-pitch SMDs but everything else as well. Then all parts can be soldered to the board in a single pass through the reflow oven. But that’s not really feasible if you’re placing all of the components manually. Our method is to place the fine-pitch ICs on your board first, then do their reflow soldering. After the board cools down you can then inspect the results and if all is well you can proceed to solder in all of the rest of the components one by one, using the fine-tipped soldering iron approach illustrated in Figs.2 & 3. You may be wondering how accu- The business end of a ‘mini spatula’ made by the author for applying a stripe of solder paste on pads for fine pitch SMDs. It’s shown here about 3x actual size. March 2008  25 So depending on the location of your fine-pitch SMDs on the board, the reflow operation can easily result in a ring of scorching on the underside of the board. The result is a totally unusable board and the SMDs won’t be able to be salvaged either. TEMPERATURE (°C) 250 225 200 183 Get an old frypan 150 SNACK OVEN TURNED OFF AT 205°C 100 50 0 0 1 2 3 4 5 6 TIME (M) NOTES: SHADED PINK AREA SHOWS RECOMMENDED TEMPERATURE PROFILE LIMITS 183°C = MELTING POINT OF TIN-LEAD SOLDER (60/40) 225°C = RECOMMENDED PEAK REFLOW PACKAGE TEMPERATURE BLACK CURVE = MEASURED TEMP PLOT OF BOARD & ICS ON 220 x 140mm x 4mm THICK ALUMINIUM PLATE, HEATED INSIDE KAMBROOK 650W KOT-150 SNACK OVEN ('BAKE' SETTING, USING BOTH ELEMENTS) Fig.6: The shaded pink area shows the reflow soldering temperature profile limits recommended by SMD manufacturers. The solid black curve shows the measured temperature plot achieved by the author using a low cost snack oven on ‘BAKE’. rately you have to place the fine-pitch IC packages in position, before reflow soldering. The answer is placed FAIRLY accurately but not fanatically so. The main thing is to make sure that every device lead is over its corresponding PC board pad, and closer to that pad than it is to any other pads nearby. If you achieve that, when the solder spheres in the paste melt and coagulate during the reflow process, surface tension and capillary forces in the molten solder will automatically ‘pull’ all of the leads into position centrally over their pads. So the idea is to carefully lower the IC package (orientated correctly, of course) into position using a ‘vacuum pickup tool’ or similar, and then nudge it gently into the correct position using a fine jeweller’s screwdriver or ‘pick tool’. Again, it’s easiest to do this under a magnifier lamp or stereo microscope, preferably one where you can rotate the board and IC until you’re happy that all leads are over their pads on the board. Once all of the fine-pitch SMDs have been placed carefully in this way, your 26  Silicon Chip board will be ready for reflow soldering. Be very careful not to bump or jar it, because the SMDs could easily be jolted out of position. Reflow soldering Now how do we do the actual reflow soldering? If you use an online search engine to track down info on reflow soldering, you’ll find that quite a few have tried doing it with an electric frypan or skillet. The basic idea is to place the PC board in the centre of the frypan, applying power until the solder paste clearly melts and flows under each SMD lead, then turn off the power and allow it all to cool down. This can work – but there is a big risk of scorching the underside of the PC board; inevitably the underside of the board must be raised to a temperature considerably higher than the melting point of solder. This board-overheating problem tends to be made worse because the heating element in the underside of most frypans is circular in shape. This produces uneven heating of the PC board, with a cooler region in the centre surrounded by a ‘ring of heat’. If you decide to try the frypan approach, please don’t use a frypan that is also used for cooking food. The fumes given off by solder paste during the reflow process are quite toxic and are likely to be absorbed by the frypan metalwork and/or Teflon coating. So the toxins may well be transferred into any food cooked in the frypan afterwards. Buy a cheap frypan specifically for the job, and mark it clearly ‘NOT TO BE USED FOR FOOD COOKING’. Because of the toxic fumes given off during reflow soldering, it’s also very desirable to do it in a well-ventilated area – preferably outdoors. This applies regardless of whether you use a frypan or some other heating device. Having read the references on the web about reflow soldering using a frypan, I decided to try it but with a slightly different approach. I bought a cheap frypan, then did a few experiments with it. To try getting around the board scorching problem, I cut a ‘heat spreader’ plate from 4mm thick aluminium sheet, and placed this in the centre of the frypan with my test board sitting on it. This did seem to make the heating fairly even but there was still a major problem. Even with the frypan’s thermostat set for maximum, the temperature on the top of the PC board never reached the melting point of solder (183°C), let alone the 215° level that is necessary to ensure good reflow. Presumably the small air gap between the bottom of the frypan and my spreader plate added too much thermal resistance. So I removed the spreader plate and tried again, with the board placed directly on the bottom of the frypan. This time the temperature on the top of the board did reach about 210°C and reflow took place, but when it all cooled down I discovered that the underside of the board had been scorched in a number of areas that had been directly over the circular heating element. siliconchip.com.au So reflow soldering with a frypan is just not worth the risk. Using a snack oven Another el-cheapo reflow technique that you’ll come across on the web involves the use of a small electric ‘snack’ or toaster oven. Almost all of these use a pair of heating elements, one at the top of the oven compartment and the other at the bottom. Whatever you’re going to heat up in the oven goes on a tray supported by a wire mesh ‘drawer’ in the centre, which is linked to the oven door so it slides in or out when the door is closed or opened. Often there’s a switch which allows you to select either the top element (‘GRILL’) or the bottom element (‘REHEAT’) or both at the same time (‘BAKE’). Each element draws about 325 watts, so the oven uses about 650W when both are used together. Since the reflow operation only involves drawing this power for five or six minutes at most, this isn’t a problem. The main advantage of using this kind of snack oven for reflow soldering is that the heating is done by infrared radiation, on the top of the board as well as from below, just like a ‘proper’ IR reflow oven. The main difference is that your board stays fixed in the oven during the whole process, rather than moving through different temperature regions on a conveyor belt. This means that you have to arrange for the reflow temperature profile to be provided in some other way. This turns out to be easier than you would think. I decided to try the snack oven approach for myself. So I bought a Kambrook KOT-150 snack oven which cost the magnificent sum of $29.95. This has no thermostat, just an electromechanical timer and the element selector switch. But the lack of a thermostat is not a problem and the timer didn’t turn out to be all that necessary either. My first test with the snack oven was to clamp a thermocouple temperature probe onto a test board, which was then placed in the small pressed tinplate tray that came with the oven. The tray was then placed on the oven’s sliding mesh drawer and the oven door closed carefully so the thermocouple lead could exit through a small gap at the top of the door. siliconchip.com.au Here’s the setup we used successfully for reflow soldering of fine-pitch SMDs. The board assembly is clamped on a 220 x 140mm plate of 4mm thick aluminium plate, with a thermocouple probe clamped to the board copper near the 100-lead device. Shortly after this shot was taken the snack oven was turned on, and then turned off again as soon as the digital thermometer reading hit 205°C. The oven was set to BAKE (both elements on) and the timer knob set to apply power for about 10 minutes. I then proceeded to take temperature measurements every 15 seconds. The resulting temperature characteristic turned out to be very close to the solid black curve in Fig.6, which also shows (shaded pink area) the reflow temperature profile limits for fine-pitch SMD IC packages recommended by larger chip manufacturers like NXP/Philips. As you can see, the warm-up characteristic is nicely within the recommended limits. By turning off the power to the snack oven when the temperature on the top of the board just reached 205°C, the board temperature coasted up nicely to a peak at 215° As soon as the temperature coasted down to about 165°C, the door of the snack oven was carefully swung down to allow the entry of more air to speed up the cooling. Both of the SMDs on this board had been reflow soldered very nicely, with no solder bridges between leads or pads. The board had not been damaged in any way, either, so I can recommend the snack oven approach. March 2008  27 and then began to coast down again. It dropped down below the 183°C solder melting point temperature about 6.5 minutes after switch-on, so after waiting about one more minute, I carefully opened the door and drawer to allow cooling to occur more rapidly. When the test board had cooled right down, I took it out of the tray and checked underneath to see if there had been any scorching. There was none at all – even the silk screening on the underside of the board showed no discolouration. Trial run Thus encouraged, I decided to carry out a reflow soldering test on another PC board. This was prepared with solder paste stripes around the pads for a fine-pitch IC and then an SMD device was carefully placed over these pads. Then I made my first mistake. In an effort to make the process a little more controlled, I drilled four 3mm holes in the oven’s tinplate tray, so the board could be fastened into it using four M3 machine screws and nuts. One of the screws was also used to attach the clamp for the thermocouple probe, to hold the probe securely in position with its bead in contact with the board’s top copper close to the SMD chip. The board and tray were carefully It’s not elegant but it works: an SMD chip baking oven, made by the author by converting a discarded blower heater. The reflector part of the heater was flattened and bent into a small rectangular oven shape, then re-attached to the front of the blower heater element (just visible through the opened front door). placed on the oven’s mesh drawer and the oven door gently closed so they slid smoothly inside. Then power was applied to the oven again, measuring the top-of-board temperature every 15 seconds as before. All went well, with exactly the same temperature profile as before. But just as the temperature reached about 200°C (just before I would turn off the power) there was a ‘PING’ sound – apparently the tinplate tray had been under stress as a result of the board being bolted inside and the stress was relieved suddenly when the temperature reached 200°. Having turned off the power as soon as the temperature reached 205°, I waited impatiently while the Ten Tips for successful DIY reflow soldering of SMDs 1. Store your solder paste in a sealed container in the fridge, to prolong its useful life. 2. Take care to apply the solder paste in a 'stripe' along the centre of the SMD lead pads on the PC board, with the stripe no more than about 1.5mm wide and (most important) very thin – no more than about 100 m, or two solder spheres. As even in thickness as you can make it, also – no lumps or voids... 3. Use a small snack oven for reflow soldering. Clamp the PC board on the top of a flat heat diffusion/support plate made from 4mm thick aluminium sheet, say 220 x 140mm in size (to fit comfortably in the snack oven). Also monitor the temperature on the top of the board near one of the SMDs, using a thermocouple probe connected to a digital thermometer. 4. Place the SMD chip(s) in position on the board carefully, with all leads as near as possible to their corresponding board pad. You don't have to be fanatical about this though: the chips will auto-locate during reflow, providing each lead is closer to its own correct pad than to the pads on either side. 5. Place the board and its support plate on the oven's slide-out drawer very carefully, so as not to bump or jolt the SMDs from their positions. Then carefully close the oven door so they slide smoothly into the oven – again without jarring. 6. Use both the upper and lower heating elements of the oven for reflow solder heating. This is usually achieved by selecting the BAKE setting. Using both elements gives more even heating, closer to that in a proper IR reflow oven. 7. Switch on the oven, monitoring the temperature on the top of board using the thermocouple probe and digital thermometer. The temperature should rise fairly smoothly, reaching the melting point of tin/lead solder (183°C) in just under five minutes. Take care not to bump or jar the oven after this. 8. As soon as the temperature reaches about 205°C, turn off the oven power without bumping anything. The temperature will continue rising, to reach a peak at around 215-220°C. It should then begin falling again. 9. Wait until the temperature drops below the melting point of solder – say down to about 165°C. Then it should be safe to open the oven door so the drawer and its contents slides out, to speed up further cooling. 10. When the board has cooled down to around room temperature, remove it from the support plate and check the solder joints on all SMD leads with an illuminated magnifier or stereo microscope. If there are any solder bridges, these can be cut away using the tip of a hobby knife or 'sucked' off using desoldering braid and a fine-tipped soldering iron. 28  Silicon Chip siliconchip.com.au temperature peaked again and crept downwards once more. Once it had dropped to about 165° I carefully opened the door, so the drawer and tray slid outwards. Then I examined the SMD chip with a magnifying glass, only to discover that stress relief ‘ping’ at 200° had caused the SMD chip to be jolted out of position. The reflow soldering had actually occurred quite nicely but with the chip and its leads in the wrong position. Bother! However, the overall result still confirmed that the snack oven was quite suitable for reflow soldering. So I decided to make a much sturdier PC board support plate, to replace the flimsy tinplate tray. The new plate was a 220 x 140mm rectangle of 4mm-thick aluminium plate and had a 3mm hole drilled near each corner, for the board hold-down clamp screws. The holes were countersunk underneath so countersink-head screws could be used to hold down the board, without producing bumps underneath the plate. This was to make sure that the plate and board could be slid smoothly around on the oven’s mesh drawer. Another board was prepared with solder paste and a fine-pitch SMD chip placed carefully in position. Then the board was clamped to the top of the new support plate, the thermocouple probe fitted and the complete assembly placed inside the oven. This time everything went really well. There were no ‘pings’, the solder reflowed nicely and when it all cooled down again a board inspection showed that the SMD chip had settled itself in the correct position and was nicely soldered. And there were no solder bridges! So we are able to report that reflow soldering of fine-pitch SMD chips can be done successfully using a low-cost snack oven like the Kambrook KOT150 shown in the pictures. Listed on the page opposite are the ten important ‘rules of thumb’ when it comes to using a snack oven for successful reflow soldering of finepitch SMD chips. If you follow these rules carefully, success is almost guaranteed. Finally, what about using a “fanforced” snack oven? Not a good idea! That fan could easily blow the SMDs SC out of position. siliconchip.com.au Footnote: About MSL rating If you’re going to be using reflow soldering for SMDs in plastic packages, you should know a bit about the way these devices are rated in terms of MSL or ‘moisture sensitivity level’. Basically, it has been discovered that SMDs in plastic packages have a tendency to absorb moisture when they’re stored in typical ‘shop floor’ or workshop conditions for any significant period of time. The degree of moisture absorption depends on a variety of factors –- including the size of the device package, the number of leads and the relative humidity level in the storage environment. The problem is that when SMDs are heated up during reflow soldering, this absorbed moisture tends to turn into steam, and build up sufficient pressure to cause cracking and other damage inside the package. It can easily damage the chip inside and/or its bonding wires, even if no cracks are visible on the outside of the package. To minimise the risk of this kind of damage during reflow soldering, chip manufacturers nowadays bake most plastic-package SMDs (especially those in fine-pitch packages) for many hours at 125°C in a very dry and inert atmosphere, to drive out any moisture. Then they are sealed in hermetic packaging (‘dry packs’), and the idea is that they should be left in this packaging until just before they’re subjected to reflow soldering. Now because this last-minute unpacking isn’t practical even for big manufacturers and in any case isn’t really necessary for some devices, semiconductor industry standards bodies like JEDEC (formerly the Joint Electron Device Engineering Council) have established a system whereby each device is given a rating to show how long it can be safely left out of its hermetic packaging in a typical 30°C/60%RH workshop or factory environment, before reflow soldering. There are eight of these MSL rating levels, ranging from MSL 1 for packages which are deemed impervious to moisture up to MSL 6 for packages which are very sensitive to moisture and must be reflow soldered within no more than six hours after being removed from their dry packs. You’ll find this MSL rating printed on the dry packs of most SMD devices in plastic packages and certainly for those in fine-pitch packages (which are almost always rated at MSL 2 or higher). Table 2 shows the significance of the various MSL levels. So what do you do if you want to reflow solder an SMD with an MSL level of 2 or higher, if you know has been out of its hermetic packaging for longer than its rated safe time? Or if it hasn’t been out for that long, but subjected to very high relative humidity? The good news is that it can be restored so it can be safely reflow soldered, by baking for about 24 hours at a controlled temperature of between 115-125°C. This can be done in a small fan driven hot-air oven, provided the device is placed in a small metal box to ensure even heating. The box should also have some small vents to allow the escape of any moisture that is released during the baking. I made up a small baking oven by converting a fan-type room heater that had been dumped at council cleanup time. The fan motor, fan and heating element were all in perfect working order, as was its thermostat switch. So all I had to do was remove these components and convert the heater case into a recirculating-air oven. Then the ‘works’ were re-installed and the thermostat tweaked to cycle the oven temperature around 118°C, which produced a rough but quite serviceable DIY baking oven for plastic package SMDs. JEDEC MOISTURE SENSITIVITY LEVEL (MSL) RATINGS MSL rating 1 Safe exposure time at <= 30°C/60%RH before reflow soldering Unlimited (non moisture sensitive) 2 1 year 2a 4 weeks 3 1 week (168 hours) 4 72 hours 5 48 hours 5a 24 hours 6 6 hours (extremely moisture sensitive) March 2008  29