PCBA Tech - PCBA process-SMT, Bonding, THT assembly process

In the PCBA electronic assembly production process, because each PCB company considers equipment cost, income, depreciation and other investment income issues, SMT production in manufacturing often uses medium and low speed placement machines; Bonding often uses ultrasonic welding machines with rotating worktables; THT often It is completed by the production method of manual plug-in manual immersion tin furnace. The effects of these mixed assembly processes on PCBA design are as follows:

1. Single-sided or double-sided layout of SMT components:

A considerable part of the PCBA used in plastic electronic products uses a mixture of SMT, COB and THT three electronic assembly production processes, so when the design is a single-sided SMT component, the use of these equipment and processes are very affordable and can be completed smoothly Designer’s product; when designing for double-sided production of SMT components, three aspects need to be considered:

1. Due to the influence of SMT equipment, the number of components arranged on one side should be as few as possible. If there are no more than five, there is no need to change the production process at a high cost, and there is no need to add solder paste with different melting points. The specific production method is:

a. First mount the side with few parts normally, and complete the reflow soldering.

b. Print solder paste on the other side. If you use a machine to print, adjust the position of the thimble on the back; if you use manual printing, you need to make a special fixture to make the PCB flat to ensure the quality of the solder paste printing.

c. After the components are mounted, put the PCB on the large aluminum plate used by Bonding, enter the reflow soldering machine, and appropriately lower the bottom surface temperature.

2. For bonding the front and back of the bare IC, leave a space of 20-30mm without SMT components to leave room for the workbench fixture of the bonding machine. Only the axis of the worktable can achieve good quality).

3. If the plug-in immersion tin furnace is needed, the thermal shock to the SMT components will exceed 200 degrees Celsius per second, and the SMT components will be damaged. At this time, it should be considered as far as possible that SMT components and THT components are arranged on different sides of the PCB separately.

2. PCB shape:

1. Generally:

Length: 50mm---460mm Width: 30mm---400mm

The most preferred PCB shape for production is:

Length: 100mm---400mm Width: 100mm---300mm

2. If the PCB is too small, it should be made into a jigsaw to facilitate machine placement and improve the efficiency of the placement machine. The jigsaw needs to be made with craft edges (if there is no craft edge, when using 0805 components, the production efficiency is low and the accuracy is poor. If the 0603 components are used, the accuracy is so poor that normal production cannot be performed.)

3. FIDUCIAL MARK must be made on the process side: a round solid copper-clad spot with a diameter of 1mm to 1.5mm.

3. Jigsaw connection:

1. V-CUT connection: use a splitting machine to split, this splitting method has a smooth section and has no adverse effects on the subsequent processes.

2. Use pinholes (stamp holes) to connect: the burrs after the breakage should be considered, and whether it affects the stable work of the fixture on the bonding machine of the COB process, and whether it affects the plug-in track and whether it affects the assembly.

Four, PCB material:

1. Cardboard PCBs such as XXXP, FR2, and FR3 are greatly affected by temperature. Due to the different thermal expansion coefficients, it is easy to cause blistering, deformation, fracture, and shedding of the copper skin on the PCB.

2. G10, G11, FR4, FR5 such glass fiber board PCB is relatively less affected by the temperature of SMT and COB and THT.

If more than two COB. SMT. THT production processes are required on a PCB, FR4 is suitable for most products from the consideration of both quality and cost.

5. The influence of the wiring of the pad connection line and the position of the through hole on SMT production:

The wiring of the pad connection line and the position of the through hole have a great influence on the soldering yield of SMT, because the improper pad connection line and through hole may play the role of "stealing" the solder, and suck the liquid solder in the reflow furnace. Go (siphon and capillary action in the fluid). The following situations are good for production quality:

1. Reduce the width of the pad connection line:

If there are no restrictions on current carrying capacity and PCB manufacturing size, the maximum width of the pad connection line is 0.4mm or 1/2 the pad width, which can be smaller.

2. It is most preferable to use a narrow connection line with a length of not less than 0.5mm (width not greater than 0.4mm or width not greater than 1/2 of the pad width) between the pads connected to the large area conductive tape (such as the ground plane and the power plane).

3. Avoid connecting wires to the pad from the side or at a corner. Most preferably, the connecting wire enters from the middle of the back of the pad.

4. Try to avoid placing through holes in the pads of SMT components or directly close to the pads.

The reason is: the through hole in the pad will attract the solder into the hole and make the solder leave the solder joint; the hole directly close to the pad, even if there is intact green oil protection (in actual production, the green oil printing in the PCB incoming material is not accurate In many cases), it may also cause heat sinking, which will change the solder joint wetting speed, resulting in tombstone phenomenon of chip components, and in serious cases, it will hinder the normal formation of solder joints.

The connection between the through hole and the land is most preferably a narrow connection line with a length of not less than 0.5 mm (a width of not more than 0.4 mm or a width of not more than 1/2 the width of the land).