PCB News - Introduction to SMT Basic Knowledge

Introduction to SMT Basic Knowledge

High assembly density, small size and light weight of electronic products. The volume and weight of chip components are only about 1/10 of that of traditional plug-in components. Generally, after SMT is adopted, the volume of electronic products is reduced by 40%~60%, and the weight is reduced by 60%~ 80%. High reliability and strong anti-vibration ability. The defect rate of solder joints is low.

Good high frequency characteristics. Reduce electromagnetic and radio frequency interference.

It is easy to realize automation and improve production efficiency. Reduce costs by 30%~50%. Save materials, energy, equipment, manpower, time, etc.

◆ Why use surface mount technology (SMT)?

Electronic products are pursuing miniaturization, and the previously used perforated plug-in components can no longer be reduced

Electronic products have more complete functions, and the integrated circuits (ICs) used have no perforated components, especially large-scale, highly integrated ICs, so surface mount components have to be used

With mass production of products and automation of production, the factory must produce high-quality products with low cost and high output to meet customer needs and strengthen market competitiveness

The development of electronic components, the development of integrated circuits (IC), the diversified application of semiconductor materials

The electronic technology revolution is imperative, chasing the international trend

◆ Why is the no-clean process applied in surface mount technology?

The waste water discharged after the product is cleaned during the production process brings pollution to water quality, the earth, and even animals and plants.

In addition to water cleaning, organic solvents containing chlorofluorohydrogen (CFC&HCFC) are used for cleaning, which also pollutes and destroys the air and atmosphere.

The residue of cleaning agent on the board will cause corrosion, which will seriously affect the quality of the product.

Reduce cleaning process operation and machine maintenance costs.

No-cleaning can reduce the damage caused by PCBA during moving and cleaning. There are still some yuan

Pieces are unbearable to clean.

The flux residue has been controlled and can be used in accordance with the product appearance requirements to avoid the problem of visual inspection of the clean state.

The residual flux has been continuously improved its electrical performance to avoid leakage of the finished product and cause any damage.

The no-clean process has passed many international safety tests, proving that the chemical substances in the flux are stable and non-corrosive

◆ Analysis of reflow soldering defects:

Solder Balls: Reasons: 1. The screen-printed holes are not aligned with the pads, and the printing is not accurate, which makes the solder paste contaminate the PCB. 2. The solder paste is exposed to too much in an oxidizing environment, and there is too much water in the air. 3. Inaccurate heating, too slow and uneven. 4. The heating rate is too fast and the preheating interval is too long. 5. The solder paste dries too fast. 6. Insufficient flux activity. 7. Too many tin powders with small particles. 8. The flux volatility is inappropriate during the reflow process. The process approval standard for solder balls is: when the distance between the pads or printed wires is 0.13mm, the diameter of the solder balls cannot exceed 0.13mm, or there can be no more than five solder balls within a 600mm square area.

Bridging: Generally speaking, the cause of the solder bridge is that the solder paste is too thin, including low metal or solid content in the solder paste, low thixotropy, easy squeezing of the solder paste, too large solder paste particles, and flux The surface tension is too small. Too much solder paste on the pad, too high peak reflow temperature, etc.

Open: Reason: 1. The amount of solder paste is not enough. 2. The coplanarity of component pins is not enough. 3. The tin is not wet enough (not enough to melt, and the fluidity is not good), and the tin paste is too thin to cause tin loss. 4. The pin sucks tin (like rush grass) or there is a connection hole nearby. The coplanarity of the pins is particularly important for fine-pitch and ultra-fine-pitch pin components. One solution is to apply tin on the pads in advance. The pin sucking can be prevented by slowing down the heating speed and heating the bottom surface more and less heating on the top surface. It is also possible to use a flux with a slower wetting speed and a high active temperature or a solder paste with different ratios of Sn/Pb to retard the melting to reduce the pin sucking tin.

◆ SMT-related technical composition

Design and manufacturing technology of electronic components and integrated circuits

Circuit design technology of electronic products

Circuit board manufacturing technology

Design and manufacturing technology of automatic placement equipment

Circuit assembly manufacturing process technology

Development and production technology of auxiliary materials used in assembly manufacturing

◆ Mounter:

Arch type (Gantry):

The component feeder and the substrate (PCB) are fixed, and the placement head (with multiple vacuum suction nozzles installed) is sending

The feeder and the substrate are moved back and forth, the components are taken out from the feeder, the position and direction of the components are adjusted, and then they are placed on the substrate. Since the placement head is installed on the arch-type X/Y coordinate moving beam, it is named after.

The method of adjusting the position and direction of the components: 1). Mechanical centering adjustment position, nozzle rotation adjustment direction, this method can achieve limited accuracy, and later models are no longer used. 2) Laser recognition, X/Y coordinate system adjustment position, nozzle rotation adjustment direction, this method can realize recognition during flight, but it cannot be used for ball grid array element BGA. 3) Camera recognition, X/Y coordinate system adjustment position, nozzle rotation adjustment direction, generally the camera is fixed, the placement head flies over the camera to perform imaging recognition, which is a little bit longer than laser recognition, but it can recognize any component, and there are some The camera recognition system that realizes recognition during flight has other sacrifices in terms of mechanical structure.

In this form, the speed of the placement head is limited due to the long back and forth movement. At present, multiple vacuum suction nozzles are generally used to take materials at the same time (up to ten) and a double beam system is used to increase the speed, that is, the placement head on one beam is taking the material while the placement head on the other beam is placed Components, the speed is almost twice as fast as the single beam system. However, in practical applications, it is difficult to achieve the conditions of simultaneous reclaiming, and different types of components need to be replaced with different vacuum suction nozzles, and there is a time delay in changing the suction nozzles.

The advantages of this type of model are: the system has a simple structure, can achieve high precision, and is suitable for components of various sizes and shapes, and even special-shaped components. The feeder has the form of belt, tube, and tray. It is suitable for small and medium batch production, and can also be used for mass production by combining multiple machines.

Turret:

The component feeder is placed on a single-coordinate moving feeder, the substrate (PCB) is placed on a worktable that moves by the X/Y coordinate system, and the placement head is installed on a turret. When working, the feeder puts the component feeder on Move to the reclaiming position, the vacuum suction nozzle on the placement head takes the component at the reclaiming position, and rotates to the placement position (180 degrees from the reclaiming position) through the turret, and passes through the position and direction of the component during the rotation. Adjust and place the components on the substrate.

The method of adjusting the position and direction of the components: 1). Mechanical centering adjustment position, nozzle rotation adjustment direction, this method can achieve limited accuracy, and later models are no longer used. 2) Camera recognition, X/Y coordinate system adjustment position, nozzle self-rotation adjustment direction, the camera is fixed, the placement head flies over the camera for imaging recognition.

Generally, there are more than ten to twenty placement heads installed on the turret, and each placement head is equipped with 2~4 vacuum nozzles (earlier models) to 5-6 vacuum nozzles (current models). Due to the characteristics of the turret, the actions are miniaturized, and the actions such as changing the suction nozzle, moving the feeder in place, taking components, component recognition, angle adjustment, table movement (including position adjustment), and placing components can all be performed in the same time period. Completed within, so real high speed in the true sense. At present, the fastest time period is 0.08~0.10 seconds for a piece of component.

This model is superior in speed and suitable for mass production, but it can only use tape-packaged components. If it is a dense-footed, large-scale integrated circuit (IC), it cannot be completed with only tray packaging. Rely on other models to work together. This kind of equipment has complex structure and high cost. The latest model is about US$500,000, which is more than three times the arch type. (Explained by PCB factory)