PCB Tech - PCB manufacturing plant produces a circuit board?

Nowadays, PCBA assembly basically uses solder to solder electronic parts on the printed circuit board. This soldering process can be through SMT (Surface Mount Technology) or wave soldering (Wave soldering). To achieve this, of course you can also use all hand welding, but that is beyond the scope of this article, and the quality of hand welding is very risky, and mass production is impossible.

Bare Board Loading

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

Solder paste printing

The first step for the printed circuit board (PCB) to enter the SMT production line is to print solder paste, which is printed on the solder pads of the parts that need to be soldered. These solder pastes are passed through the high-temperature reflow oven later. It will melt and solder the electrical parts to the circuit board.

SMT

Solder paste inspector (option)

Since the quality of 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.

Pick and Place speed machine

Here, some small electronic parts (such as small resistors, capacitors, and inductors) will be placed 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 the speed of the patch 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.

Pick and Place general machine

Also known as "slow speed machine", here will be some relatively large electronic parts, such as BGA ICs, connectors... etc. These parts need to be positioned accurately, so the alignment is very important. Before the film, I used a camera to confirm 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.

Hand place component or visual inspection

When all the parts are printed on the circuit board and go through the high temperature reflow oven (reflow), a check point is usually set up to pick out the defects of the patch offset or missing parts... etc., because after the high temperature furnace, if If there is a problem, it is necessary to 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 traditional parts of DIP or some special reasons cannot Parts operated by the placement/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 reflow. Sometimes it is because the shielding frame is marked on the part, which will cause the AOI to be unable to check after the reflow furnace. Solderability.

Reflow furnace (Reflow)

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.

Optical Inspection Solderability (AOI, Auto Optical Inspection) Option

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, it is impossible to judge the quality of parts such as false welding, BGA weldability, resistance value, capacitance value, and inductance value, 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, and I hope to achieve 100%, so I have to make a trade-off. .

Unloading (unloading)

After the board is assembled, it will be returned to the magazine (magazine), 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 AOI’s bad results

Touch up

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.

PCB Open/Short Circuit Test (ICT, In-Circuit Test)

The purpose of ICT setting is mainly 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 high temperature reflow Whether the function after the furnace is damaged, wrong parts, missing parts... etc.

PCB function test (Function test)

PCBA function test

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

Panel (assembly board de-panel)

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) and four-in-one (4 in 1). …Wait. After all the assembly operations are 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.