PCBA Tech - SMT processing plant PE engineer instructions

The following is an introduction to the notices for PE engineers of SMT processing plants:

1. What are the characteristics of solder paste?

2The soldering process of solder paste components?

3. How to optimize process parameters? Such as the optimization of the profile curve?

4. The influence of solder paste, process parameters, and equipment on printing solder paste? How to improve?

5. The production of Profile DOE? How to make the CPK of the placement machine?

6. How does SPI prove that the system is OK and credible, and is the data accurate?

7. Verification of poor incoming materials, the plating of component pins? How many bad samples will be taken?

8. The formation mechanism of IMC? What effect does the thickness of IMC have on welding? IMC

How thick is the layer and what is its range?

9. What is the main basis for engraving the steel mesh? What are the area ratio and width-to-thickness ratio?

10. How to control the amount of Udfiller glue? How to calculate the amount of glue? What is the calculation formula?

11. How to evaluate the single board in DFM? If there are many double-panel components, it can only be designed as a double-panel, and there are many components on the second side. Another component can only be placed on the first side. Can this component be designed on the first side? What is the basis?

12. What are the elements of DFM in veneer technology?

13. IMC layer analysis? What is the main analysis in the IMC layer?

14. Slicing experiment?

15. How to do solder paste evaluation?

16. Why does the first side component drop when the component passes through the reflow oven twice? Is there a corresponding calculation formula to prove that the component will not drop?

Answer: 1. What are the characteristics of solder paste?

Viscosity, fluidity, thixotropy, the melting point is usually lead 183, lead-free 217, etc.

2. What is the soldering process of solder paste components?

Can be divided into 4 stages: heating, constant temperature, reflux, cooling

Heating up: The printed and patched PCB board enters the reflow soldering, and the temperature rises slowly from room temperature.

The heating rate is controlled at 1-3°C/S.

Constant temperature: By maintaining a stable temperature, the flux in the solder paste will work and evaporate in an appropriate amount.

Reflow: At this time, the temperature rises to the highest, the solder paste is liquefied, the alloy is formed between the PCB pad and the solder end of the part, and the soldering is completed. The time is about 60S, which is determined by the solder paste.

Cooling: Cool the welded board, and the cooling speed can be controlled well to obtain beautiful solder joints, such as ROHS 6-7°C/S.

3. How to optimize process parameters? Such as the optimization of the profile curve?

Generally, three steps of preset, measurement, and adjustment are required to obtain the best parameters. Take the temperature curve as an example. First, preset the reflow speed and temperature of each temperature zone according to the type of solder paste, PCB thickness, etc., and then use a furnace temperature tester to measure the actual temperature curve of the PCB board, and then refer to the past Experience and the conventional process of solder paste soldering require analysis,

Repeatedly adjust and verify the preset temperature and walking speed to obtain the most suitable curve file.

4. What are the influences of solder paste, process parameters, and equipment on printing solder paste? How to improve?

First of all, the solder paste may have poor moldability, thixotropy, fluidity, etc. due to its composition ratio, particle size or irregular use, which may cause collapse, short circuit, and less tin during printing. Process parameters such as printing pressure, squeegee speed, squeegee angle, etc. will cause insufficient tin, sharpening, irregular molding, or even tin after solder paste. The hardware is mainly scraper and steel

Net tension, opening size, opening shape, surface roughness, stencil thickness, and support and fixation of the PCB by the printer cause poor printing. In short, there are many factors that determine the quality of solder paste printing. In actual production, according to the actual problems, the real cause of the bad is analyzed and then adjusted to the best.

5. What is the production of Profile DOE? How to make the CPK of the placement machine?

DOE: Design of Experiments. A statistical method for arranging experiments and analyzing experimental data. The production of Profile DOE can be completed in the following steps: 1. According to the company's "Reflow Soldering Operation Guide", select the test indicators: such as the set speed, the set temperature of each temperature zone, etc. 2. Refer to the key points of the analysis experiment and determine the most suitable test position on the board as the experimental position. 3. Expected (take historical experience as a reference) the results (such as bridging, virtual welding, etc.) that will appear at the experimental location, waiting for statistics in the list.

4. After the preparation is completed, repeat several sets of experiments, statistical results, analyze the results, and determine the best parameters.

5. Verify the final profile, make a summary report, and complete.

The CPK of the placement machine is the index of the placement machine's accuracy\process capability. There are formulas to calculate, but now all are automatically calculated by software (such as Minitab). Equipment CPK production can be used for experimental design: the accuracy of the equipment is corrected, and the standard jig is used to carry out multiple mounting tests with different heads, different positions and different angles, and then measure the position deviation, and compare the deviation of the obtained multiple sets of data Enter the CPK calculation software to obtain the CPK value. Generally, when the standard CPK is greater than 1, the process capability is normal.

6. How does SPI prove that the system is OK and credible, and is the data accurate?

The question is a bit unclear. Three understandings of SPI: there is an SPI system (software process improvement), a company that sells overall solutions in the United States, and an SPI device. I don't know the first two very well. I only know that the SPI device is a device for testing solder paste printing. The surface shape of the object is obtained through ternary color illumination, combined with red laser scanning, and intensive sampling. Then automatically identify and analyze the solder paste area, and calculate the height, area, volume, etc. I like its ability to automatically learn boards, automatically generate coordinates and export EXCEL files. Ha, maybe the SPI root of the problem

I didn't know it. .

7. Verification of poor incoming materials, the plating of component pins? How many bad samples will be taken?

Defective incoming materials require IQC according to relevant standard documents such as engineering approval samples or IPC

General standards or negotiated standards, etc., shall be randomly inspected; the quantity can be ordered according to GB/T2828, and the sample quantity can be determined according to the AQL acceptance standard. The plating layer of the component pins is generally pure tin, tin-bismuth or tin-copper alloy, which is only a few microns thick. The end structure of the chip component is: inner palladium-silver electrode, middle nickel barrier layer, and outer lead-tin layer.

8. The formation mechanism of IMC? What effect does the thickness of IMC have on welding? How thick is the IMC layer and what is its range?

IMC (Intermetallic compound) is formed by the migration, penetration, diffusion, and bonding of metal atoms during welding. It is a thin layer of alloy-like alloys, which can be written with molecular formulas, such as between copper and tin: benign Cu6Sn5, malignant Cu3Sn, etc. If there is normal welding, the IMC layer will appear, and the IMC layer will age and thicken, and it will stop until it encounters the barrier layer. It will cause welding embrittlement and difficulty in tinning again. The general thickness is 2-5μm.

9. What is the main basis for engraving the steel mesh? What are the area ratio and width-to-thickness ratio?

The opening of the steel mesh is mainly based on the Gerber file of the PCB or the actual PCB. The width and area of the steel mesh have been basically fixed after long-term practice and experience accumulation. When the pad is extremely large, the middle frame grid should be used to ensure the tension. Width and area are the basic requirements for opening steel mesh: area ratio = opening area ÷ hole wall area generally greater than 0.66 (ROHS 0.71) width-thickness ratio = opening width ÷ template thickness generally greater than 1.5 (ROHS 1.6)

10. How to control the amount of Udfiller glue? How to calculate the amount of glue? What is the calculation formula?

Udfiller is talking about bottom filling, to ensure the long-term reliability of smt chip assembly. Weighing method can be used to control the amount of glue used: take 20 boards as samples, weigh and calculate the weight difference between before and after glue is attached, and then calculate the amount of glue used per board. The formula can be: (the weight of the sample with glue G2-the weight of the sample before glue G1) ÷ the number of samples N = the amount of glue used for a single piece G. You can also think of various methods by yourself: (weight in the plastic bottle before use-after use Weight) ÷ production quantity, the amount of veneer can also be calculated, and the process loss can be counted.