In the SMT process, solder paste printing, placement machine operation, and reflow furnace welding are all key processes. The incoming inspection is the first condition to ensure the quality of surface assembly. The quality of components, printed circuit boards and surface assembly materials directly affects the assembly quality of surface assembly boards.

Therefore, there must be a strict incoming inspection and management system: the electrical performance parameters of the components and the solderability of the soldering terminals and pins, productivity design of printed circuit boards and solderability of pads, quality of surface assembly materials such as solder paste, surface mount glue, stick solder, flux, cleaning agent, etc.

To prevent PCBA bursting with CTI> 600, the key is to reduce the moisture absorption of printed boards during the manufacturing, storage and use of printed boards.

1. Inspection method

The inspection methods mainly include visual inspection, automatic optical inspection (AOI), X-ray inspection and ultrasonic inspection, online measurement, and functional measurement.
1.1 Visual inspection refers to the method of inspecting the assembly quality directly with the naked eye or with the help of a magnifying glass, a microscope, and the like.
1.2 Automatic optical inspection (AOI), mainly used for process inspection: solder paste printing quality inspection after printing machine, placement quality inspection after placement, and post-weld inspection after reflow furnace, automatic optical inspection is used instead of Visual inspection: X-ray inspection and ultrasonic inspection are mainly used for solder joint inspection of BGA, CSP and Flip Chip.
1.3 The online test equipment uses special isolation technology to test the resistance of the resistor, the capacitance of the capacitor, the inductance of the inductor, the polarity of the device, and parameters such as short circuit and open circuit. Failure, and can display and print out errors and failures.
1.4 Functional test: Used for the electrical function test and inspection of surface-mounted boards. The functional test is to use the surface assembly board or the unit under test on the surface assembly board as a functional body to input electrical signals, and then detect the output signal according to the design requirements of the functional body. Most functional tests have diagnostic procedures to identify and determine faults. However, the functional testing equipment is relatively expensive. The simplest functional test is to connect the surface assembly board to the corresponding circuit of the device and power it on to see if the device can operate normally. This method is simple and requires less investment, but it cannot automatically diagnose the fault.
Which method is used depends on the specific conditions of the SMT production line of each unit and the assembly density of the surface assembly board.

2. Surface Mounted Components (SMC / SMD) Inspection

The main testing items of components: solderability, coplanarity and usability, should be sampled by the inspection department. The solderability of components can be tested by stainless steel tweezers and the component body can be immersed in a tin pot at 235 ° C ± 5 ° C or 230 ° C ± 5 ° C, taken out at 0.2s or 3s 0.5s, and inspected under a 20x microscope welding at the soldering end. 90% soldering at the soldering end of the component is required. As a processing workshop, the following visual inspections can be done:

2.1 Visually or use a magnifying glass to check whether the soldered ends of the components or the surface of the pins are oxidized and free of contamination.
2.2 The nominal value, specifications, model, accuracy, and dimensions of the components should be consistent with the product process requirements.
2.3 The pins of SOT and SOIC cannot be deformed. For multi-lead devices QFP with a lead pitch of 0.65mm or less, the pin coplanarity should be less than 0.1mm (can be optically detected by the placement machine).
2.4 For products requiring cleaning, the marking of the components does not fall off after cleaning, and it does not affect the performance and reliability of the components (visual inspection after cleaning).

3. Printed Circuit Board (PCB) Inspection

3.1 The pad pattern and size of the PCB, solder mask, screen, and via settings should meet the design requirements of the SMT printed circuit board. (Examples: check whether the pad asking distance is reasonable, whether the screen is printed on the pad, whether the via hole is made on the pad, etc.).
3.2 The overall dimensions of the PCB should be consistent, the overall dimensions of the PCB, positioning holes, reference marks, etc. should meet the requirements of the production line equipment.
3.3 PCB allowed warpage size:
1.Up / convex: max. 0.2mm / 5Omm, max. Length 0.5mm / length direction of the whole PCB.
2.Down / concave: max. 0.2mm / 5Omm, max. Length 1.5mm / length direction of the whole PCB.
3.4 Check PCB for contamination or moisture

4. PCB packaging storage

After the PCB manufacturer has finished manufacturing the PCB with high CTI value, it bakes and dehumidifies the baking process, and then uses 2 high-vacuum double-film packaging to increase the sealing of the printed circuit board packaging. Finally, a desiccant is placed in the large package.
The packaged PCB storage should be stored in a normal temperature and low humidity environment and placed on the platform or a suitable rack in the original packaging. Avoid heavy pressure and prevent plate deformation caused by improper storage. The storage period is 3 months.
PCBA assembly manufacturers should check whether the packaging is intact, confirm the effective storage period, and the condition of the humidity card in the inner packaging to meet the requirements. Prevent the use of expired PCBs.

5. PCBA board production

First, the baking process of the PCB light board is dehumidified before placement, and then put into production to complete the placement-plug-in-test-packaging and shipment, and the entire production process is required to be completed within one week in the production planning arrangement.
The reflow soldering and wave soldering are set according to their operation instructions. The warm-up rate cannot be too fast. Set the peak value of the temperature to no more than 245 degrees Celsius. The higher the temperature, the greater the risk of board explosion. For manual welding repair welding, use a temperature-controlled soldering iron, set the temperature to 350 degrees Celsius, and control the welding time to 2 to 3s to prevent local overheating and the formation of potential defects.

For PCBA production with CTI> 600, the key to prevent board explosion is to do a good job of prevention and quality control. The production management of PCB with CTI> 600 is the cornerstone of preventing board explosion. In addition, in the PCBA assembly production, the thermal stress is controlled to prevent the thermal stress factors induced by the explosion of the board.