PCB News - How does an electronic manufacturing plant produce a piece of circuit board

How does an electronic manufacturing plant produce a piece of circuit board

Nowadays, the assembly of circuit boards is basically soldering electronic parts to the printed circuit board through solder. This soldering process can be through surface mount technology (SMT, Surface Mount Technology) or wave soldering (Wave soldering) To achieve this, of course, you can also weld all by hand.

Because wave soldering is rarely used in products, this article only introduces surface mount soldering (SMT) for circuit board assembly soldering. Basically, the current SMT process is a one-stop operation, that is, the empty board is placed on the SMT production line, and finally the board is discharged and assembled on the same production line.

The following process will briefly introduce the process from empty board loading to board assembly, as well as the quality assurance post process:

Bare Board Loading

The first step of assembling the circuit board is to arrange the bare boards neatly and place them on the magzine. The machine will automatically send the boards into the SMT assembly line one by one.

The first step for the printed circuit board(PCB) to enter the SMT production line is to print solder paste. To be honest, this is a bit like a girl applying a mask on her face. Here, the solder paste will be printed on the solder pads of the parts that need to be soldered. Above, these solder pastes will melt and solder the electrical parts on the circuit board when they pass through the high-temperature reflow furnace.

Since the quality of the solder paste printing is related to the quality of the soldering of the subsequent parts, some SMT factories will first use optical instruments to check the quality of the solder paste printing after the solder paste printing, if there is a poorly printed board. Break it down, wash off the solder paste on it and reprint it, or use a repair method to remove the excess solder paste.

Here, some small electronic parts (such as small resistors, capacitors, and inductors) will be printed on the circuit board first. These parts will be slightly stuck by the solder paste that has just been printed on the circuit board, so even if the speed of the printing is Very fast, almost like a machine gun, the parts on the board will not fly away, but the large parts are not suitable for use in the fast machine, which will slow down the speed of the small parts that were originally hit. Secondly, I am afraid that the parts will shift from the original position due to the rapid movement of the board.

Also known as slow speed machine, here will be some relatively large electronic parts, such as BGA IC, connector... etc. These parts need to be positioned more accurately, so the alignment is very important. In the past, I used a camera to check the position of the parts, so the speed is much slower. Due to the size of the parts here, there may not always be tape-on-reel packaging, and some may be in tray (Tray) or tube (tube) packaging. But if you want the SMT machine to eat pallets or tubular packaging materials, you must configure an additional machine.

Generally, traditional pick and place machines use the principle of suction to move electronic parts, so there must be a flat surface on the top of these electronic parts for the suction nozzle of the pick and place machine to pick up the parts. However, some electronic parts cannot have a flat surface for these machines. At this time, it is necessary to order special nozzles for these special-shaped parts, or attach a layer of flat tape to the parts, or wear flat surfaces. Cap.

When all the parts are printed on the circuit board and go through the high temperature reflow oven (reflow), usually a check point is set to pick out the shortcomings of the offset or missing parts... etc., because the high temperature furnace is passed. Later, if you still find a problem, you must move the soldering iron (iron), which will affect the quality of the product, and there will be additional costs; in addition, some larger electronic parts or DIP traditional parts or some special reasons, The parts that cannot be operated by the punching/placement machine will also be manually placed here.

In addition, the SMT of some mobile phone boards will also design an AOI before the reflow furnace to confirm the quality before the reflow. Sometimes it is because the shielding frame is marked on the part, which will cause the AOI to be unable to check the solder after the reflow furnace. sex.

The purpose of the reflow furnace (reflow) is to melt the solder paste and form a common gold (IMC) on the part feet and the circuit board, that is, solder the electronic parts on the circuit board, and the temperature profile of the rise and fall of the temperature will be It often affects the quality of the soldering of the entire circuit board. According to the characteristics of the solder, the general reflow furnace will set the preheating zone, wetting zone, reflow zone, and cooling zone. With the current lead-free process of SAC305 solder paste, its melting point is about It is about 217°C, which means that the temperature of the reflow furnace must be at least higher than this temperature to remelt the solder paste. In addition, the maximum temperature should not exceed 250°C, otherwise there will be many parts deformed because they cannot withstand such high temperatures. Or melt.

Basically, after the circuit board passes through the reflow furnace, the entire circuit board assembly is completed. If there are exceptions for hand-soldered parts, the rest is to check and test the circuit board for defects or malfunctions.

Not every SMT production line has an optical inspection machine (AOI). The purpose of setting up an AOI is because some circuit boards with too high density cannot be used for subsequent open and short circuit electronic testing (ICT), so AOI is used instead, but Because AOI has blind spots for optical interpretation, for example, the solder under the part cannot be judged. At present, it can only check whether the part has tombstone or side, missing parts, displacement, polarity direction, tin bridge, empty solder, etc. However, the quality of parts such as fake welding, BGA weldability, resistance value, capacitance value, and inductance value cannot be judged, so there is no way to completely replace ICT so far.

Therefore, if only AOI is used to replace ICT, there are still some risks in terms of quality, but ICT is not 100%. It can only be said that the test coverage rate is compensated for each other. I hope to achieve 100%, so I have to make a trade-off. .

After the board is assembled, it will be returned to the magzine, which has been designed to allow the SMT machine to automatically pick and place the board without affecting its quality.

Finished product visual inspection (Visual Inspection)

Regardless of whether there is an AOI station or not, the general SMT line will still set up a circuit board visual inspection area to check whether there are any defects after the circuit board is assembled. If there is an AOI station, it can reduce the visual inspection personnel. Quantity, because we still need to check some places that AOI can't read, or check the bad AOI.

Many factories will provide a visual inspection template at this station, which is convenient for visual inspectors to inspect some important parts and their polarity.

Touch up

If some parts cannot be produced by SMT, touch-up hand-welded parts are required. This is usually placed after the finished product inspection to distinguish whether the defect comes from SMT or the subsequent process. When recombining parts, use a soldering iron (iron) and a solder wire. During soldering, the soldering iron, which is maintained at a certain high temperature, touches the foot of the part to be soldered until the temperature rises to a temperature sufficient to melt the tin wire, and then add tin The wire melts, and after the tin wire cools, the parts are soldered to the circuit board.

There will be some fumes when hand welding parts, and these fumes will contain a lot of heavy metals. Therefore, the operation area must be equipped with fume exhaust equipment, and try not to let the operator inhale these harmful fumes. It needs to be reminded that some parts will be arranged in a later stage of the process due to the needs of the process.

Circuit board open/short test (ICT, In-Circuit Test)

The main purpose of ICT setting is to test whether the parts and circuits on the circuit board are open or short. In addition, it can also measure the basic characteristics of most parts, such as resistance, capacitance, and inductance, to determine that these parts have undergone a high-temperature reflow furnace. After the function is damaged, wrong parts, missing parts... etc.

Circuit testing machines are divided into advanced and elementary machines. The elementary test machines are generally called MDA (Manufacturing Defect Analyser). Its function is to measure the basic characteristics of electronic parts and judge open and short circuits as mentioned above.

In addition to the high-end test machine including all the functions of the first-level model, it can also send power to the board under test, start the board under test and execute the test program, the advantage is that it can simulate the function of the circuit board under actual power-on conditions The test can partially replace the function test machine (Function Test) that follows. But a test fixture of this high-end test machine can probably buy a private car, which is 15 to 25 times higher than a preliminary test fixture, so it is generally used in mass-produced products. More suitable.

Circuit board function test (Function test)

function test

Functional testing is to make up for the deficiencies of ICT, because ICT only tests the open and short circuits on the circuit board, and other functions such as BGA and products have not been tested, so it is necessary to use a functional testing machine to test all functions on the circuit board.

Panel (assembly baord de-panel)

Conference Board

General circuit boards will undergo panelization to increase the efficiency of SMT production. There are usually so-called several-in-one boards, such as two-in-one (2 in 1), four-in-one (4 in 1).. . Wait. After all the assembly work is completed, it must be cut (de-panel) into single boards. Some circuit boards with only single boards also need to cut off some extra board edges (break-away).

There are several ways to cut the circuit board. You can design a V-cut (V-cut) using a blade cutting machine (Scoring) or direct manual folding (not recommended). More precise circuit boards will use a path splitting cutting machine. (Router), it will not harm the electronic parts and circuit boards, but the cost and working hours are longer.