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What are the dangers of SMT patch processing?
Time:2021-10-08 Read: 1592

Precautions for the use of solder paste in SMT patch processing:

1. Storage temperature: It is recommended that the storage temperature in the refrigerator is 5℃-10℃, and should not be lower than 0℃.

2. Outgoing principle: The principle of first-in first-out must be followed. Do not first-in-last-out, which may cause the solder paste to be stored in the freezer for a long time.

3. Defrosting requirements: After taking out the solder paste from the freezer, thaw at room temperature for more than 4 hours, and the bottle cap cannot be opened to thaw at room temperature.

4. Production environment: It is recommended that the workshop temperature be 25±2℃, and the relative humidity should be used in an environment of 45%-65%.

5. Used old solder paste: try to use the solder paste within 12 hours after unsealing. If you need to save it, please keep the container clean. After the sealing is completed, put it back into the freezer for storage.

6. The amount of printing paste: the amount of printing solder paste placed on the stencil should not exceed 1/2 of the height of the squeegee during printing, so as to ensure frequent observation and increase the number of times.

SMT patch processing technology needs to pay attention to when printing operations:

1. Scraper: The quality of the squeegee is preferably a steel squeegee, which is conducive to the release of the film printed on the pad and the formation of the solder paste.

Squeegee angle: manual printing is set to 45°-60°; machine printing is set to 60°.

Printing speed: manual 30-45mm/min; printing machine 40mm-80mm/min.

Printing environment: The temperature is 23±3℃, and the relative humidity is 45%-65%RH.

2. Steel mesh: The thickness of the steel mesh and the shape and proportion of the opening are selected according to the requirements of the product.

QFP\CHIP: 0402 CHIP and center distance less than 0.5mm need to use laser to open holes.

Testing the steel mesh: The tension test of the steel mesh should be carried out once a week, and the tension value is above 35N/cm.

Cleaning the stencil: When printing 5-10 pcs of PCB board continuously, wipe it with dust-free net paper once. Try not to use rags.

3. Cleaner: IPA solvent: alcohol solvent and IPA should be used to clean the steel mesh, and chlorine-containing solvents cannot be used, because it will destroy the composition of the solder paste and affect the entire quality.

The issues that must be paid attention to about the hidden dangers of SMT patch processing quality are as follows:

1. The missing parts (bumps) of the capacitor will not affect the function at all, but only affect the life and effect;

2. The quality of BGA welding cannot be seen directly (bubble, unmelted, offset, etc.);

3. Circuit failure caused by micro short circuit;

4. ESD causes damage to the life of electronic components or functional decline;

5. IC/BGA is damp and causes poor function or potential failure.

1. Be sure to pay attention to electrostatic discharge measures during SMT chip processing. It mainly includes the design of the chip processing and the re-established standards, and the corresponding treatment is carried out in order to be sensitive to electrostatic discharge during SMT chip processing. And protective measures are very critical. If these standards are not clear, you can refer to the relevant documents to learn.

2. SMT chip processing must fully comply with the above evaluation standards of welding technology. When welding, common welding and manual welding and other related measures are usually used, while those required for SMT chip processing For welding technology and standards, you can refer to the welding technology evaluation manual. Of course, some high-tech SMT patch processing plants also carry out 3D construction of the products to be processed, so that the effect after processing will reach the standard, and its appearance will be more beautiful.

3. After the SMT chip processing and welding technology is the cleaning measure, the cleaning also needs to be strictly in accordance with the standard, otherwise the safety after the SMT chip processing will not be guaranteed. Therefore, when cleaning, the type and nature of the cleaning agent are required, and the integrity and safety of the equipment and process need to be considered during the cleaning process.

1. SMT workshop environment requirements

1. Requirements for plant load-bearing capacity, vibration and noise

1) The bearing capacity of the plant floor should be greater than 8KN/m2

2) Vibration should be controlled within 70dB, and the larger value should not exceed 80dB

3) The noise should be controlled within 70dBA.

2. Power

Generally, single-phase AC220 (220±10%, 0/60Hz) and three-phase AC380 (380±10%, 50/60Hz) are required. The power of the power supply should be more than twice the power consumption.

3. Air source

According to the requirements of the equipment, the pressure of the air source can be configured. The air source of the SMT chip processing plant can be used, or an oil-free compressed air machine can be configured separately. The general pressure is greater than 7kg/cm2. Clean and dry purified air is required, so the compressed air needs to be degreasing, dusting, and water removal treatment. Use stainless steel or pressure-resistant plastic pipes for air ducts.

4. Exhaust

Reflow soldering and wave soldering equipment need to be equipped with exhaust fans. For all hot blast stoves, the lower flow rate of the exhaust duct is 500 cubic feet per minute (14.15m3/min).

5. Lighting

The ideal illuminance in the workshop is 800~1200LUX, at least not less than 300LUX. When the illuminance is low, install local lighting in the inspection, repair, measurement and other work areas.

6. Working environment

The workshop should be kept clean and sanitary, free of dust and corrosive gases. The production workshop should have cleanliness control, and the cleanliness should be controlled at: 500,000.

The ambient temperature of the production workshop is preferably 23±3, generally 17-28, and the relative humidity is 45%-70%RH.

Set up a suitable temperature and humidity meter according to the size of the workshop, carry out regular monitoring, and be equipped with facilities for adjusting temperature and humidity.

Two, SMT anti-static requirements

1. Requirements for electrostatic work area

1) SMT workshop floor: Anti-static polyvinyl chloride (PVC) floor and epoxy resin dust-proof artesian floor anti-static floor can be used in two ways.

2) At the entrance of the SMT workshop, there is a place where a dedicated person can change anti-static clothes and a wardrobe;

3) Operators: uniform anti-static clothes, gloves, shoes, hats, and anti-static wristbands at each station;

4) The entrance of the SMT workshop must be equipped with personnel static protection safety test crossings. Only qualified personnel can enter, otherwise they cannot enter.

5) Production line: Each station has a corresponding electrostatic interface, which is connected to the electrostatic system of the entire SMT workshop; each workbench uses an anti-static mat;

6) Regularly measure and record the surface resistance of the ground, desktop, turnover box, etc.

7) Non-production items, such as tableware, cups, bags, woolen clothes, newspapers, rubber gloves, etc., are prohibited on the workbench in the electrostatic safety zone.

8) Wear an anti-static wrist strap during operation, and measure whether the wrist strap is effective every day

9) When testing electrostatic sensitive components, one piece should be taken from the box, tube, and plate, one piece should be tested, and one piece should be placed.

10) The power-on test must follow the power-on sequence, that is, the sequence of low voltage→high voltage→signal voltage, and the power-off sequence is the opposite. At the same time, note that the polarity of the power supply cannot be reversed, and the power supply voltage must not exceed the rated value.

11) Warehouse: Raw materials are stored in anti-static material cabinets, and corresponding anti-static measures are taken;

12) After production, finished products and semi-finished products are stored in anti-static turnover boxes

13) The unpacked PCB board of the printing station shall be stored in an anti-static turnover box;

14) Thermo-hygrometer control: SMT workshop has at least 2 thermo-hygrometers (specifically arranged according to the size of the workshop), and the temperature and humidity conditions are confirmed and recorded every two hours. The temperature control range is 20~26, and the humidity control range is 45%~70%RH. It is forbidden to operate electrostatic sensitive components in an environment lower than 30%RH.

2. Frequency requirements for quality control in anti-static zone

Regularly maintain and check the effectiveness of anti-static facilities

3. Requirements for transportation, storage and use of humidity sensitive components

1. Transportation requirements

1) Do not fall to the ground during transportation, and do not leave the package arbitrarily

2) If the packaging needs to be changed, a container with anti-static properties must be used.

2. Storage requirements

1) Store in a vacuum packaging bag with desiccant;

2) Store in a dry box (humidity <10%RH)

Warehouse requirements for non-drying cabinet storage

1) The warehouse must have temperature and humidity control, temperature: 23±3, humidity: 30%~40%RH

2) The material rack where humidity sensitive components are placed must be grounded, and the friction voltage is controlled to be less than 100V

3) Moisture-sensitive components that have not been used up after opening the package must be stored in a drying cabinet or vacuumed, and have a tracking card. The temperature of the drying cabinet: 23±3, humidity: <10%RH

3. Requirements for use

1) The package can only be opened 10 minutes before the release of the material, and the moisture sensitive card must be checked when opening the package. The dots indicate that blue is normal, and red is damp.

2) Production is interrupted during SMT patch processing and production. The production stop time is more than 5 hours, and humidity sensitive components must be returned to the warehouse for dry storage; if the components are unpacked and left at room temperature for 12 hours, they must be dried at RH10%. It can be used again after 12H (or baking at 120 degrees 2H\60 degrees 4H).

3) Establish tracking cards for humidity sensitive components.

4) Disposal of humidity sensitive components after unpacking:

The exposure time of humidity sensitive components in production and use should be implemented according to the storage conditions and standards corresponding to different humidity sensitivity levels in the table on the next page; if the incoming material warning label has been specified and the requirements are compared with the requirements in the table If it is more stringent, it will be implemented in accordance with the conditions specified on the warning label.

For components with humidity sensitivity levels of 2a-5a, if you need to open the original package to take some components, the remaining components must be stored in a dry box immediately, and a moisture-proof component unpacking time tracking card should be attached.

5) When writing, erasing and information protection operations on EPROM, the writer/eraser should be fully grounded, and an anti-static bracelet should be worn.

6) Operators of assembly, welding, repairing, debugging, etc. strictly follow the requirements of electrostatic protection.

7) Printed circuit boards that have passed the test and inspection should be sprayed once with an ion spray gun before packaging to eliminate the static charge that may accumulate.

What are the bad welding methods in SMT patch processing

1. Randomly choose the soldering iron tip without considering the appropriate size. In the process of patch processing, the selection of the size of the soldering iron tip is very important. If the size of the soldering iron tip is too small, the residence time of the soldering iron tip will be prolonged, and the solder will not flow sufficiently and lead to cold solder joints. If the size of the soldering iron tip is too large, the joint will heat up too quickly and damage the patch. Therefore, the selection of the appropriate soldering iron tip size should be based on the three criteria of the correct length and shape, the correct heat capacity, and the larger the contact surface but slightly smaller than the pad.

2. The temperature setting is incorrect. Temperature is also an important factor in the soldering process. If the temperature is set too high, it will cause the pad to lift up, the solder will be overheated and the circuit patch will be damaged. Therefore, setting the correct temperature is particularly important for the quality assurance of patch processing.

3. Improper use of flux. It is understood that many workers are accustomed to using too much flux in the process of patch processing. In fact, this not only does not help you have a good solder joint, but also causes the reliability of the lower solder joint, which is prone to corrosion. , Electronic transfer and other issues.

4. The process of welding the heating bridge is not appropriate. The soldering thermal bridge in SMT chip processing is to prevent the solder from forming a bridge. If this process is not operated properly, it will cause cold solder joints or insufficient solder flow. Therefore, the correct soldering habit should be to place the soldering iron tip between the pad and the pin, and the tin wire is close to the soldering iron tip. When the tin melts, move the tin wire to the opposite side, or place the solder wire between the pad and the pin. , The soldering iron is placed on the tin wire, and the tin wire is moved to the opposite side when the tin is melted; in this way, a good solder joint can be produced and the chip processing will not be affected.

5. Excessive force is applied to the lead welding during SMT patch processing. Many workers in SMT chip processing plants believe that excessive force can promote the heat conduction of the solder paste and promote the soldering effect, so they are used to pressing down forcefully during soldering. In fact, this is a bad habit, which can easily lead to problems such as warping, delamination, depression, and white spots on the PCB. Therefore, it is completely unnecessary to use excessive force during the soldering process. In order to ensure the quality of the patch processing, only the soldering iron tip needs to be lightly touched to the pad.

6. Improper transfer welding operation. Transfer soldering refers to adding solder to the tip of the soldering iron first, and then transfer to the connection. Inappropriate transfer soldering will damage the tip of the soldering iron and cause poor wetting. Therefore, the normal transfer soldering method should be that the soldering iron tip is placed between the pad and the pin, and the soldering wire is close to the soldering iron tip. When the tin is melted, the soldering wire should be moved to the opposite side. Place the tin wire between the pad and the pin. Place the soldering iron on the tin wire and move the tin wire to the opposite side when the tin melts.

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