SMT basic introduction

              

1. Smt features

High assembly density, small size and light weight. The volume and weight of patch components are only about 1/10 that traditional insertion components. Generally after adopted SMT, the electronic products volume is reduced by 40% to 60% and the weight is reduced by 60% to 80%. High reliability and strong vibration resistance.

The solder joint defect rate low.

The high frequency characteristics. Reduced electromagnetic and radio frequency interference.

Easy to automate and increase productivity, reduce costs by 30% to 50%. Save on material, energy, equipment, manpower and time and so on.

 

2. Why we use surface mount technology (SMT)?

2.1 Electronic products pursuit miniaturization and previous perforated plugin components can not be reduced.

2.2 Electronic products have more complete functions, and the used  integrated circuits(IC) have no perforated components, especially large-scale, high integrated ICs. So the SMT has to be used.

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

2.4 The development of electronic components and the development of integrated circuits(ICs), and the multiple application of semiconductor materials.

 

3. Why does the no-clean process apply in surface mount technology?

3.1 In the process of production, the waste water from the back of the product can bring the pollution of the water quality,the earth and animals.

3.2 In addition to water cleaning, organic solvents containing chlorofluorocarbons(CFC&HCFC) are used for cleaning, as well as pollution and destruction of air and atmosphere.

3.3 The residue of cleaning agent can cause corrosion on the board, which seriously affects the quality of products. 

3.4 Reduce cleaning operation and maintenance cost. 

3.5 No cleaning can reduce the damage caused by PCBA in the process of movement and cleaning.Some of the components remain unwashed. 

3.6 The flux residues have been controlled and can be used to match the appearance of the product to avoid visual inspection of the cleaning status. 

Residual flux has continuously improved its electrical properties to avoid leakage of the finished product, resulting in any damage. 

3.7 The no-wash process has passed international safety tests to prove that the chemicals in the flux are stable and non-corrosive.

 

4. Reflow Defect Analysis:

Solder Balls: Causes: 1. The printed hole is not aligned with the pad. The printing is inaccurate, which makes the solder paste stain the PCB. 

2. Excessive exposure of solder paste in oxidizing environment and too much moisture in the air. 

3, heating is not accurate, too slow and uneven. 

4. The heating rate is too fast and the preheating interval is too long. 

5.Solder paste done too fast. 

6. Flux activity is not enough. 

7. Too much small particles of tin powder. 

8. The flux is not properly volatile during reflow. The process approval standard for solder balls is: When the distance between pads or printed conductors is 0.13mm, the diameter of solder balls cannot exceed 0.13mm, or more than five solder balls cannot appear in the range of 600mm square. The

The bridging (Bridging): In general, the cause of bridging is due to the fact that the paste is too thin, including low levels of metal or solids in the solder paste, low whirl-resistance, easy squeezing of solder paste, too much solder paste particles, and flux The surface tension is too small. A lot of solder paste on the pad, the peak reflow temperature is too high. The

Open: Reason: 1. There is not enough solder paste. 

2. Coplanarity of component pins is not enough. 

3. Tin wet enough (not enough melting, fluidity is not good), solder paste is too thin caused by loss of tin. 

4.Pin suction tin (like wick grass) or near the connection hole. The coplanarity of the pins is particularly important for fine-pitch and ultra-fine-pitch pin elements. One solution is to pre-tock the pads. Pin lead absorption can be prevented by slowing the heating rate and heating the bottom surface and heating the top surface less. It is also possible to use a flux that has a slower wetting speed, a higher active temperature flux, or a solder paste that blocks the melting with a different ratio of Sn/Pb to reduce pin attraction.

 

5. SMT-related technology components

Electronic components, integrated circuit design and manufacturing technology

Circuit Design Technology for Electronic Products

Circuit board manufacturing technology

Automatic placement equipment design and manufacturing technology

Circuit Assembly Manufacturing Process Technology

Development and production technology of auxiliary materials used in assembly and manufacturing