Surface mount technology is called SMT for short, which is an intellect - intensive, technology - intensive and comprehensive engineering science technology; SMT is mainly in the process of PCB, PCBA and soldering. And solder paste plays an important role in quality and efficiency in SMT process. Paste printer, a apparatus which plays a vital role in SMT line with the most highest technology and properties demands, it is made up of stencil, scraper, printing table and so on. After positioning the stencil and printed circuit board, which put pressure on scraper, and shift the scraper to be rolling the solder paste, filling it in the stencil opening. What’s more, taking advantage of the solder paste’s thixotropic and adhesion to transfer in the printed circuit board with mesh.
There are some industry experts analyzing that more than 60% of the defects in SMT fabrication are due to solder paste printing.
What affect the solder paste printing?
The main factors affecting solder paste printed covered:
1.The stencil’s quality
Firstly, the quality of stencil, which is the most important factor for printed solder paste. The thickness of stencil and opening dimension make a decision on printing of solder paste. Excessive printing will lead to bridging, otherwise it will be pseudo soldering or insufficient solder paste. There are some factors also affecting the quality of stripping, which the hole wall of stencil whether is smooth or not and the shape of stencil opening. However, the tension needs to be higher than 30N, on the contrary, it will result in uneven printing or insufficient solder paste.
2.Solder paste’s quality
Secondly, the quality of solder paste. The shelf life at normal temperature will make an effect on printing quality.
3.Printed process parameter.
There are some restricted connection between the speed of scraper, stress, the angle of scraper and stencil and the bonding of solder paste. Only in controlling the parameter can it improve the quality of printing.
It also makes an effect on precision and re-printing precision of printer as printing the fabrication with high density and small spacing.
As you know, excessive temperature will reduce the bonding of solder paste. High humidity will absorb the moisture in the air,however on the contrary, it will accelerate the volatilization of solvent in the solder paste. And there will be some defect of pin hole for soldering spot when the dust is mixed in the solder paste under the temperature.
As the most important SMT in the solder paste printing, which is mainly in accurately coating the solder paste on the pad where solder paste is required on PCB. It is necessary to print with stencil in the printing process, every coin has two sides, but the stencil makes a direct effect on printing quality. Now the normal metal stencil, it’s made up of thinner metal board with pin hole, and solder paste is easy to flow through the hole to PCB in the open hole. In fact, there is no clearance between metal stencil and PCB, it has a long shelf life with one hundred thousand times.
What are the types of SMT stencil?
Etching: it’s popular in the early stage, and it’s suitable for the open hole with more than 20mil, etching accuracy is +/-1mil.
Laser: a manufacturing process, it’s more accurate to position with +/-0.3mil.
Electroform: it’s suitable in the printing under 5mil, which is mainly in semiconductor fabrication.
An aperture is formed on the stencil by coating the resist on the metallic foil, exposing the pattern on the two sides of metallic foil with pin positioning sensitive tool, and then etching the metallic foil with double process.
Advantage: to be moulding just one time, it works quickly with a cheap price.
Disadvantage: it’s easy to be the hour glass shape( insufficient etching); or the opening dimension is more bigger( excessive etching); there are some objective factors making a great effect, such as experience、medicament、film, it is complicated to manufacture with large error, and chemical etch not does well in fine pitch process; there will be polluted and it’s bad for environmental protection in stencil manufacturing process.
Being laser cutting from customer’s original Gerber data, there is a great position accuracy and re-fabrication in stencil process. Laser technology is the unique process that the stencil is allowed to rework.
Advantages：a high accuracy for data manufacturing, and little impacts on objective factors; trapezoidal opening is good for stripping; laser cutting can be controlled; it has a suitable price to use.
Advantage：it need to cut one by one with slow manufacturing.
A increasing process with developing photo resist on a substrate of opening, and then plating the stencil around the photo resist one by one.
Advantage: there is a smooth aperture wall, especially do well in fabrication of ultra fine pitch stencil.
Advantages: it’s difficult to control and there will be polluted in the manufacturing process; what’s more, electroform has a long time to fabrication with a high price.
What does the stencil work in SMT solder?
With the development of SMT to high density and ultra-high density assembly, stencil plays an important role in PCBA services. The stencil plays a role in printing the solder paste with semi-liquid and semisolid to PCB, now a lot of popular PCB use SMT technology but expect power panel, there are so many pads with label on PCB that is no via, however, the hole on the stencil is the component assembly pad on PCB, printing the solder paste with semi-liquid and semisolid to PCB by the hole of stencil is what manual printing solder paste,and mounting devices with mounter,then reflow solder.
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