PCBA Tech - SMT cleaning process after welding and dust-free workshop requirements

SMT chip processing and cleaning process after welding

SMT chip processing and cleaning after soldering refers to the use of physical action and chemical reaction methods to remove the flux residues and SMT patches remaining on the surface layer of the surface layer assembly board after SMT reflow soldering, wave soldering and manual soldering. The pollutants and impurities caused by the process of processing and assembling process are harmful to the surface layer of the assembling board.

1. The activator added to the flux and welding sound contains a small amount of western compounds, acid or salt, and the residual debris will cover the surface layer of the solder joint after welding. When the electronic device is powered on, the ions of the remaining impurities will migrate to the electrical conductor with the opposite polarity, which may cause a short circuit in severe cases.

2. At this stage, the halides and chlorides in the more common fluxes have very strong activity and hygroscopicity, which will corrode the substrate and solder joints in the humid environment, which will reduce the insulation resistance of the substrate surface layer and cause electromigration., When the situation is serious, it will conduct electricity, causing a short circuit or open circuit.

3. Products with special requirements such as high-standard military products, medical products, and meters need to be treated with three-proofing. The standard before the three-proofing treatment has a high degree of cleanliness, otherwise it will be exposed to relatively harsh environmental conditions such as hot flashes or high temperatures. Cause serious consequences such as electrical performance degradation or failure.

4. Due to the speed shift of the residual debris after welding, the test probe contact is not good during the online test or the function test, which is prone to false test.

5. For high-standard products, some defects such as thermal damage and spalling cannot be exposed due to the occlusion of residual debris after welding, resulting in missed inspection and affecting reliability. At the same time, many impurities also affect the appearance of the PCB substrate and the commercial nature of the board.

6. Residual debris after soldering will affect the connection reliability of high-density, multi-I/O connection point array chips and flip-chips.

Second, the requirements of SMT patch processing for dust-free workshops

General SMT patch processing and production workshops have the following requirements for a dust-free environment. First, the load-bearing capacity, vibration and noise requirements of the plant. The load-bearing capacity of the plant road surface should exceed 8KN/m2; the vibration should be controlled within 70dB, the highest value No more than 80dB; noise should be controlled within 70dBA; switching power supply generally requires single-phase AC220 (220±10%, 0/60Hz), three-phase AC380 (380±10%, 50/60Hz), and the output power of the switching power supply More than double the functional loss.

The next step is the air source requirements of the SMT patch processing workshop. The air source pressure is equipped according to the equipment requirements. The air source of the factory can be used, or an oil-free compressed air machine can be independently equipped, and the general pressure exceeds 5kg/cm2. Clean and dry purified air is required, so the compressed air must be degreasing, dusting, and sewage treatment. Use stainless steel plates or pressure-resistant plastic hoses as gas pipelines. There are also requirements for an exhaust system, and the reflow oven and wave soldering machine must be equipped with exhaust fans. For all hot blast stoves, the minimum flow value of the exhaust system pipe is 500 cubic inches/minute (14.15m3/min).

The ideal number of lighting in the SMT patch processing workshop is 800~1200LUX, at least not less than 300LUX. When the lighting is low, install some lighting in areas such as inspection, maintenance, and precise measurement. SMT patch processing and production workshop must maintain daily cleaning, no dust, no corrosive vapor. The production workshop needs to have cleanliness control, and the cleanliness control is: 500,000; the working temperature of the production workshop is 23±3 degree Celsius as the best, generally 17～28 degree Celsius, and the air humidity is 45%～70%RH; The specification and size of the production workshop are set with a suitable thermometer and hygrometer, which are monitored regularly and equipped with equipment for adjusting temperature and humidity.

The above is the requirements of SMT chip processing for dust-free workshops