Elecronics printed Circuit Board (PCBA) assembly OEM EMS manufacturing Process Design Considerations, posted by China Supplier Topscom Technology.
Today's circuit board assembly, basically through the solder to the electronic parts on the printed circuit board, the welding process can be surface bonding, (smt,surface Mount Technology) or wave soldering (wave soldering) to achieve, Of course, you can also use all hand welding, but that is not in the scope of this discussion, and the quality of hand welding is very high risk, can not be produced in large quantities. Because the wave soldering has very few products in use, so this article will first introduce to surface welding (SMT) to do PCB assembly welding. Basically now the SMT process is a one-stop operation, that is, put the empty board to the beginning of SMT production line, to the final board out of the Assembly completed, are in the same line. The bottom process will simply be introduced from the empty board to the Board assembly to complete, there are quality assurance after the process: empty board loading (Bare Board Loading) circuit board Assembly The first step is to empty the board (Bare Board) arranged neatly, and then put on the rack (magzine) above, The machine will be automatically a piece of the handle into the SMT production line. Printing solder paste (soder paste printing) circuit board into the SMT production line of the first step to the printing of solder paste (solder paste), it is really a bit like a girl in the face smear mask, it put the solder paste in the need to weld the parts of the pad, These solder paste will melt and weld the electrical parts to the circuit board at the back of the high-temperature reflow oven. Solder paste Inspection (Solder paste Inspector) (option) due to the solder paste printing is good or bad, so some SMT factory for quality stability, will be first in the solder paste printing after the use of optical instruments to check the quality of solder paste, if there is poor printing of the board will be knocked out , wash off the solder paste in the printing, or use the repair method to remove the extra solder paste. Quick Punch (Pick and place speed machine) Here are some of the smaller electronic parts (such as small resistors, capacitance, inductance) on the board, these parts will be slightly glued to the freshly printed solder paste, so the speed of the pieces is very fast, almost like a machine gun, The board on the above parts are not yet flying, but the hit type parts are not suitable for use in the speed machine, which will drag the original hit the speed of small parts, and to be afraid of parts will be due to the rapid movement of the board offset the original position. Pan-type punching machine (Pick and place General machine)
Here is a number of relatively large electronic components, such as BGA ICS, Connectors (connector), these parts need to be more accurate location, so the position is very important, the speed is much slower. Some of the parts here are not necessarily tape-packaged (Tape-on-reel), some may be pallets (Tray) or tubular (tube) packaging. Conventional pick (machine) use the principle of suction to move electronic parts, so the top of these electronic components must be left with a plane to the suction nozzle, but some electronic parts can not have the plane left to these machines, This time you need to order special suction nozzle to these profiled parts. Hand pendulum part or visual inspection (hand place component or visual inspection) and all the parts are on the circuit board. Before the high-temperature reflow furnace, a checkpoint is usually set up to pick out whether any parts are offset or dropped ... And so on, because after the high temperature furnace, if you still find problems, you must move the iron, there will be additional costs in terms of quality, and other parts that are too large for electronic components or dip, or for special reasons that cannot be manipulated by a punching machine, and where they can be manually laid in hand.The welding Furnace (reflow) Reflow furnace is designed to melt solder paste and form a common gold (IMC) in the parts of the foot and circuit board, that is, welding electronic parts on the circuit board, the temperature of the rise and Fall of the curve (temperature profile) often affect the quality of the entire circuit board welding, According to the characteristics of the solder, the general welding furnace will set the preheating zone, infiltration zone, weld Zone, cooling zone, with the current lead-free process of SAC305 solder paste, the melting point of about 217 ℃, that is, the furnace temperature at least at this temperature to be able to melt the solder paste, the other highest temperature is best not more than 250 ℃, Otherwise there will be a lot of parts because there is no way to withstand that high temperature and deformation or melting. Basically after the circuit board after the welding furnace, the entire circuit board assembly even if completed, if there are hand welding parts exception, the rest is to check and test the board has no defect or poor function of the problem. Optical Inspection tin (AOI, Auto optical inspection) option does not necessarily have an optical inspection machine (AOI) for each SMT production line, and the purpose of setting up the AOI is because some of the high-density circuit boards are not able to carry out subsequent short circuit electronic testing (ICT), So replace it with Aoi, but since Aoi has a blind spot for optical interpretation, for example, the solder at the bottom of the parts can not be judged, currently only able to check the parts have tombstone (tombstone) or side, missing parts, displacement, polar direction, Tin Bridge, air welding, but can not judge the false welding, BGA solder, resistance value, capacitance value, Inductance value and other parts quality, so so far there is no way to completely replace ICT. So if you use only AOI to replace ICT, there are still some risks in the product, but ICT is not 100%, can only say that each other's test coverage rate, they have to do a trade-off (unloading) board group will be completed in the back to the rack (magzine) , these racks have been designed to allow the SMT machine to automatically take the board without affecting its quality. Finished product inspection (Visual inspection) Whether or not set up Aoi Station, the general SMT line will be set up the visual inspection area of the circuit board, in order to check the circuit board after the completion of the assembly is any bad, if there are AOI stations can reduce the number of inspection personnel, Because we still have to check some Aoi can not be interpreted to the place, or check Aoi hit down the bad. Many factories will provide visual inspection Board (inspection template) at this station to facilitate inspection personnel to check the polarity of some key parts and parts. Part back (touch up) If there is a hand-welded part to be back, it is usually placed after the finished product inspection, to distinguish whether the defect is from SMT or from the next part of the process. Use soldering iron (iron) and such (solder wire) in the back-laminating parts, welding will maintain a certain high temperature soldering iron contact to the parts of the foot, until the temperature to increase enough to melt the temperature of the tin wire, and then add tin to melt, waiting for the cooling of the tin wire will be welded parts on the circuit board. Hand welding parts will have some smoke generation, these smoke mostly belong to heavy metals, operation area must set up smoke discharge equipment, try not to let operators inhale these harmful smoke. To be reminded, some parts are resumed after the process is scheduled to be in a later process due to the processing requirements.
Circuit board Open/short test (ICT, in-circuit test) The purpose of ICT is to test circuit board parts and circuits have open and short circuit, it can also measure the basic characteristics of most parts, such as resistance, capacitance, inductance value, To determine whether these parts after high-temperature reflow furnace function is damaged, wrong parts, missing parts ... Wait Circuit test machine is divided into advanced and first-order machine, the first stage of the test machine we generally call MDA (manufacturing Defect Analyser), its function is to say the basic characteristics of electronic components measured and open short-circuit. In addition to all the features of the first order machine, the higher-order test machine can also power to the test board, and to activate the test board to carry out the testing program, the advantage is that the board can simulate the actual power-on of the situation of the functional test, can be part of the replacement of the functional test machine (function testing). But a test fixture of this high order test machine may buy a private car, compared to a first-order test fixture to be higher than 15~25 times, so generally used in a large number of products are more suitable. Circuit Board functional test (function test) functional testing is to make up for the lack of ICT, because ICT only Test circuit board on the open short-circuit, other such as BGA and product features are not tested, so you need to use the functional test machine to test all the features on the board. Plate (Assembly Baord De-panel) General circuit board will be a puzzle (panelization) to increase the efficiency of SMT production, there will usually be called "several in one" board, such as the two in one (2 in 1), four in one (4 in 1) ... Wait When all the assembly work is completed, it will be cut (de-panel) into veneer, and some only board circuit board also need to cut some superfluous edge (Break-away). There are several methods of cutting, design V-groove (v-cut) using a blade cutting machine (scoring) or direct manual folding plate (not recommended), the more sophisticated circuit board will use the path cutting Machine (Router), compared with the electronic parts and circuit boards will not hurt, but the cost and work longer.