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PCBA Tech
SMT patch processing operation detailed process
PCBA Tech
SMT patch processing operation detailed process

SMT patch processing operation detailed process

2021-11-07
View:73
Author:Downs

There are roughly four major steps in the operation process of SMT patch processing: solder paste printing → component placement → reflow soldering → AOI.

Further breakdown:

Loading → solder paste printing (side A) → placement (side A) → visual inspection before the furnace → reflow oven → solder paste printing (side B) → placement (side B) → inspection before the furnace → reflow oven → after the furnace Inspection → Sub-board → Download → FQA inspection → Production inspection → FT test → BT test → CIT test

The following is a detailed introduction to each step:

1. Loading

Material loading means that after the factory receives the customer's BOM, the corresponding program will be written, and then the material number and item name will be listed in the corresponding machine. At this time, the warehouse can match the materials of the project to be produced in advance according to the plan, and then the material production personnel will put the materials into the corresponding machine according to the material number set in the machine. After the production material personnel have received the materials, the inspectors will coordinate to check whether there are any inconsistencies in the material numbers, and sign on the material loading records. PQA will randomly check the material loading conditions during line inspection.

1. When a tray of materials is used up, especially when picking materials from the warehouse, there are labels on only one layer, and they will be placed in the wrong position when the personnel are not paying attention. At present, all materials must be confirmed by both parties before they can go online.

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2. When the incoming material number is handwritten, there is a quality risk. The handwritten material number itself may be wrong, and the person checking the material number may mistake the material number for other material numbers.

3. When it is difficult to distinguish the materials during production throwing and recycling, they are all in the throwing box. When throwing and recycling, it is necessary to define the time to be used up.

4. When the materials are relatively small and need to be patched separately, the corresponding personnel are required to confirm when reposting the label.

Second, solder paste printing

The solder paste must be warmed up before use. After unpacking, record the unsealing time and mix it evenly before it can be used online. At present, the control method of solder paste control printing is an important parameter that records the relationship between the printing results and cannot be deviated outside the defined range, that is, the squeegee pressure, demolding speed, demolding distance, printing speed, automatic cleaning frequency, automatic cleaning speed, etc. OP's requirement is two hours of cleaning, one time manual cleaning of the steel mesh, and there is a cleaning record.

The final monitoring method for the effectiveness of the solder paste machine is to measure whether the thickness of the solder paste is within the standard range, and use the value of CPK to monitor the effectiveness of MPM/DEK. However, the monitoring method for the solder paste offset is only the OP looking at the magnifying glass, and if the board has problems such as continuous soldering or offset after reflow soldering, the problem of solder paste printing will be investigated back.

1. The solder paste is not used as required.

2. The printing problem is not reported to the relevant personnel for adjustment in time.

3. Although the height of the solder paste after printing meets the requirements of the range, but when CPK1.67 or 7 consecutive points are on the side of the center line, the staff failed to feedback the problem in time. Even after the problem is reported, the relevant craftsman is not clear about how to adjust .

Three, patch

The SMT component feeder and substrate (PCB) are fixed. The placement head moves back and forth between the feeder and the substrate to take the component out of the feeder, adjust the position and orientation of the component, and then place it on the substrate.

1. When some holes in the suction nozzle are blocked, the appearance and color of the material are different, it will cause the machine to throw the material and so on.

2. When the material tape is not placed horizontally, the material tape breaks, and the viscosity is too high, which is attributed to the supplier.

3. When the incoming material is placed in the material belt inconsistently, or the incoming material does not match the size of the fault plate, it will also affect the quality of the patch.

4. The personnel inspecting in front of the furnace are interested in supervising the problem. If it is above 0603, a slight deviation will not affect the quality of the product, but if it is a 0.5PIN component, in principle, no deviation is allowed.

Fourth, reflow soldering

In the process of hot air reflow soldering, the solder paste needs to go through the following stages to volatilize the solvent; the flux removes oxides on the surface of the soldering parts; the solution of solder paste flows again to cool and solidify the solder paste.

1. Preheating zone

Preheat the PCB and components to achieve equilibrium, while removing the moisture and solvent volatilization in the solder paste. It is relatively mild, and the thermal shock to the components is as small as possible. Too fast heating will cause damage to the components, such as cracking of the multilayer ceramic capacitor. At the same time, it will also cause solder spatter, so that solder balls and solder joints with insufficient solder will be formed in the non-soldering area of the entire PCB.

2. Insulation area

Ensure that the solder can be completely dried before reaching the reflow temperature, and at the same time, it also plays the role of flux activation to remove the metal oxides in the components, pads, and solder powder. The time is about 60 to 120 seconds, depending on the nature of the solder.

3. Reflux area

The solder in the solder paste causes the gold powder to melt and flow again, replacing the liquid solder to wet the pads and components. This wetting effect causes the solder to expand further, and the wetting time for most solders is 60 to 90 seconds. The temperature of reflow soldering must be higher than the melting point temperature of the solder paste, and generally must exceed the melting point temperature by 20 degrees to ensure the quality of reflow soldering. Sometimes this area is divided into two zones, namely the melting and reflow zone.

4. Cooling zone

The solder solidifies as the temperature decreases, so that the components and the solder paste form a good electrical contact. The cooling rate must not be too different from the preheating rate.

Five, sub-board

At present, we use a splitter, which uses rotary cutting, but the factory sometimes cuts the sub-board with manual scissors due to production capacity. When it is necessary to cut by hand, prepare a document to inform the OP of the cutting order, and when the cut is completed, the effect of inspection is to prevent the phenomenon of breaking the last part of the board by hand.

Six, test

For workstations such as download, BT, FT, etc., check whether the version used is consistent with the customer's work order, and whether the fixture used for the job and the power supply are implemented in accordance with the requirements. This is a software test, and the factory can keep the data for future reference. As for the CIT station, because we currently have a lot of test items, many items in the test process require human judgment to pass or not. This item is prone to missed tests. In the case of tight production capacity, other departments will propose random tests and other suggestions. of.

In the current test control method, the code of the CIT tester is written on the board, so that people in the latter stage can judge whether the board has done CIT test, but in general, because the personnel judge whether it is passed, if the personnel do not understand the following , Misdetection may occur. The bad judgment of the test is mostly done manually, and it would be more accurate if the automatic mode can be introduced.

Seven, inspection

The inspection standard is the secondary standard of IPC-610D. At present, due to the inconsistency of SMT factory personnel's mastery of the standard. That is, a 10 times magnifying glass is sufficient for inspection, but for doubtful places, a better magnifying glass is needed for arbitration. For example, the reliability and standard of soldering tin of L-shaped pins are defined as the root of the pin. The current equipment cannot see the soldering on the root and back end of the L-shaped pin, it is just whether it is bad based on the experience of the personnel.