Dejin Computer Technology

Electronics Design & Manufacture

This paper introduces the SMT surface welding process and the matters needing attention

00. Shop Floor Tracking Number:

SMT production line of work must be in the nude version (bare PCB) jump in production on the serial number, serial number in this group in the main track their production record, record if used correctly, it can track the board it what the Date code of the electronic materials and from which MPN, it also allows us to track the whole process for any abnormal conditions or bad repair. Of course, these are the premise, want to have what harvest must have paid.


At present, there are about 4 schemes for the serial number printing of the circuit board. At present, the company in the front will recommend the use of "radium carving" technology, which has fewer side effects and is beautiful.


In addition, some companies that can not keep up with the pace will not necessarily print the serial number in the blank stage, but wait until the completion of the plate will make the serial number, serial number import in the more front end of the process, the more complete the production resume can be traced.


01. Bare Board Loading:

The first step in PCB assembly, of course, is to load the bare board into the SMT pipeline. By far the most common technique is to empty plate and tidy after overlapping sequence, placed on shelves, then like the printer paper transport mechanism from the top of the board by the pieces into the SMT production line conveyor belt, but this kind of operation in the process of pushing for some boards sometimes scratch the surface of the problem, so it is sometimes empty version will also be placed on points in the rack (magazine), when the machine heap send so there would be no scratch problem, was just one more into the rack.


Each of these processes has a Sensor as an eye transmitted to the computer, which then determines when to push the board and gives instructions on when to stop the board.


02. Soldering Paste Printing:

The first job to get a Printed Circuit Board into the line of SMT flow was to use solder paste. To be honest, this is a bit like a girl applying a facial mask on her face.


This step will reduce the solder paste through steel plate (font) printed in PCB need to welding parts welding pad/bonding pad (pad), the location of the solder paste volume can affect the welding quality of follow-up, the solder paste will in the subsequent SMT process flows through the "back to the welding furnace (Reflow Oven)" in the high temperature melting and solidification again in the process of the electronic parts welded on the circuit board.


There are two main purposes of using "solder paste" to combine electronic parts with PCB:

1) stick the electronic parts to the circuit board before soldering, so that they will not be deviated due to PCB's movement or vibration. That's why it's a paste.

2) fix the electronic parts to the PCB by welding at high temperature after the back welding, so that they will not fall off during the use of the end user, and achieve the purpose of electronic signal transmission.


In addition, some people will use rubber film board/rubber paper board to replace the solder paste during the new product trial, which can increase the efficiency of SMT mixer and reduce waste.


03. Soldering Paste Inspection:

Solder Paste printing quality will directly affect the subsequent parts of welding throw out good or bad, so now the most advanced of the EMS factory for its stable quality, will first after the Solder Paste printing extra set an Optical instrument, used to check the quality of the Solder Paste printing, this instrument is referred to as the "Solder Paste Inspection machine (Solder Paste Inspector, SPI)", its principle and AOI (Auto Optical Inspection), after checking if found that poor Solder Paste printing board can pick out first, Wash off the solder paste and reprint the solder paste, or remove the excess solder paste by repairing.


The SPI is important because solder paste if after back weld solidification, solder paste solidification parts have welding problem is discovered after it is necessary to use the soldering iron heavy industry, or scrap, if can be found before the early solidification solder paste printing and improve or solve problems, you can greatly reduce the production scrap rate and reduce the cost of repair.


04. Chip Placement, Pick and Place speed machine:

Circuit boards of electronic components generally divided into active components (IC parts) and passive components (Inductors, Capacitors, Resistors, etc), and this kind of SMD passive components (such as Small resistance, capacitance and inductance), also known as "Small Chip" size is generally Small, and generally only two endpoints need to be welded, so in such Small parts will be put on the circuit board the location of the relative accuracy is low, So the design of a fast/high speed Chip Placement machine (Chip Placement machine), which will generally have a number of nozzle, and the speed is very fast, like a revolver gun, can hit several parts a second.


At this time of the solder paste paste of advantages are evident, because when playing a suction mouth usually only down action, and parts of the accuracy of registration will have to rely on the board moved, these have been playing in the circuit boards of electronic parts would be just printed on the PCB solder paste stick (which is also one of the reasons why the solder paste to make paste), so even if play a very fast (board moving party often fast), the board of the above parts also not be off flying, but large parts or have a certain weight of parts is not suitable for a machine to process with fast and, so will drag down the original play fast speed of small parts, Secondly, I am afraid that the parts will shift from the original position due to the rapid movement of the board.


Depending on the number of small parts on the circuit board, an SMT production line will generally have 1 ~ 4 sets of fast parts.


05. Pick and Place general machine:

This kind of general use of the smash-up machine is also known as the "slow machine". It can almost be applied to all SMD parts SMD parts need, but because it is not the pursuit of speed, but the accuracy of the parts, so the slow machine is generally used to play some large or heavy or multi-pin electronic parts, such as BGA integrated circuit, connector, card reader, shielding frame/cover... Such as, because these parts need to be more accurate location, so its alignment and Angle adjustment ability becomes very important, after the pick will use a camera to take a look at the appearance of the part, and then adjust the position and Angle of the part will be placed (placement), so the overall speed is relatively slow a lot.


Because of the size of the electronic parts, there may not always be tape-on-reel, but some may be Tray or tube. But for SMT machines to eat pallets or tubes of packaging, an extra machine is usually required.


Conventional play a piece machine (pick and place machine) is using the principle of suction to take put electronic parts, so these electronic parts on the top must keep a clean piece of plane to play suction nozzle to absorb a part of a machine, but some electronic components is unable to have a plane to these machines, then you need custom-made special profiled suction nozzle to these parts, or labeled with a surface layer of adhesive tape on the parts, or wearing a flat cap.


06. hand place component or visual inspection:

When all the parts are printed on the circuit board, a check point is usually set up to pick out offset or missing parts before entering the reflow. After the high temperature furnace, if the welding problem is found again, the soldering iron must be moved, which will affect the quality of the product, and there will be additional costs; Other parts that cannot be operated by the pavers, such as larger electronic parts or DIP/THT traditional parts or for special reasons, are also placed manually.


In addition, SMT of some mobile phone boards will set a "pre-furnace AOI" in front of the back welding furnace to confirm the quality of SMT before the back welding. Another situation is certain board in the SMT phase will directly (Shielding - can) "shields" welded on the circuit board, once the shield is put on the circuit board can't through the AOI or visual inspection method to check the SMT and soldering quality, and suggested that has this kind of situation must enlarge set a more "before the furnace, AOI" placed before the "shield" patch.


07. Reflow:

Back welding (reflow) is the purpose of the solder paste melting and formation of nonmetallic compound (IMC) in parts between the feet and circuit board, and electronic components is welded on the circuit board, the curve of the temperature rise and fall (temperature profile) tend to affect the whole circuit board welding quality, according to the properties of the solder, the general will be set back to welding furnace preheating zone (pre - heat), infiltration area (soak), back to the welding area (reflow), cooling zone (cool) to achieve the best effect of solder.


The melting point of SAC305 solder paste in lead-free process is about 217℃, that is to say, the temperature of the rewelding furnace should be at least higher than this temperature to re-melt the solder paste. In addition, the highest temperature in the rewelding furnace should not exceed 250℃, otherwise many parts will deform or melt because they cannot withstand such a high temperature.


Basically after the circuit board through the back welding furnace, the whole circuit board assembly even if completed, if there are hand welding parts exception, the only thing left is to check and test the circuit board for defects or dysfunction.


08.   (AOI, Auto Optical Inspection) Option:

Although "AOI" after the furnace has almost into the current standard of SMT, but do not necessarily each SMT production line will be equipped with "optical inspection (AOI), set up one of the purposes of" after the furnace AOI "because some of the high density circuit board cannot be effective for subsequent open short circuit test (ICT), so use AOI to replace, but because of AOI for optical interpretation, has its innate blind spots, such as solder cannot be checked at the foot of the parts, the location of the adjacent high parts will have the shadow effect cannot effectively check, At present, AOI can only inspect visible parts for tombstone, lateral, missing, displacement, polarity direction, tin bridge, air welding, etc., but it is not possible to determine the quality of false, BGA, resistance, capacitance, inductance, etc. Some AOI cannot even inspect the side solder of QFN or Castellated terminations.


So far, AOI cannot completely replace ICT and FVT.


Therefore, if only AOI is used to replace ICT, there will still be a great risk in terms of quality, but ICT is not 100%. It can only say that the test coverage rate is made up for each other, and it is expected to achieve 100% test coverage rate, so I have to make a trade-off.


09. Unloading:

When the board is assembled, it will be returned to the magazine, which has been designed to allow the SMT machine to automatically take and place the board without affecting its quality. However, attention should be paid to the upper and lower spacing of different boards to avoid the problem of electronic parts bumping.


10. Visual Inspection:

Whether or not the establishment of AOI station, the general SMT line will set up a circuit assembly board visual inspection area, confidence or insufficient ah! Objective to check whether there are any defects after the circuit board assembly is completed. If the AOI station is set, the number of visual inspection personnel can be reduced, because some AOI can not be read, or AOI recheck the defects. With the current technology, AOI still has a certain misjudgment rate.


Many factories will provide visual inspection templates at this station to facilitate visual inspectors to check the weldability and polarity of some key parts.


11. Parts Touch up or wave weld parts:

If some parts have no way to use SMT to patch, then (touch - up) will need to hand welding parts or the use of "wave soldering (wave soldering)" or "choose welding" to traditional hole welding parts, this step is usually placed in SMT product inspection, the purpose is to distinguish the disadvantage is that from SMT or after SMT process.


"After the" electronic components to use the soldering iron (iron) and tin wire (solder wire), will remain in a certain temperature when welding the welding head touches the soldering and welding parts of foot pad, until the temperature rise enough to melt tin wire temperature, then add the tin wire melting, quickly remove the welding head in order to reduce soldering temperature, after waiting for tin wire cooling solidification would bring parts welded on the circuit board.


When hand welding parts, there will be some smoke, which will contain a lot of heavy metals, so the operating area must be set up smoke exhaust equipment, try not to let the operator to breathe these harmful smoke.


It should be noted that some parts will be reprocessed later in the process due to process requirements.