Stencil is one of the most important tools in SMT processing. Today we will introduce the its classification and impact on Assemble SMT processing.
According to the production process of SMT, stencil can be divided into: Laser Template, Electropolishing Template, Electroforming Template, Ladder Template, etc..
Laser Template (Laser Stencil)
The laser stencil template is currently the most commonly used template in the SMT stencil industry and its features below:
* Direct use of data files to reduce the production error
* High opening position accuracy: full error ≤ ± 4μm
* The opening of the Laser template has a geometric shape that facilitates the printing of the solder paste.
Electropolishing Template (E.P. Stencil)
The Electropolishing template is post-treated by electrochemical means after laser cutting to improve the open cell walls. Its characteristics are:
* The hole wall is smooth, especially suitable for QFP/BGA/CSP with ultra-fine pitch.
* Reduce the number of wiping of SMT templates and greatly improve work efficiency.
Electroformed Formwork Stencil (E.F.Stencil)
In order to comply with the requirements of short, small, light and thin electronic products, ultra-fine volume and ultra-fine pitch components are widely used. As a result, the SMT steel mesh industry has put forward higher requirements for printing templates, so electroforming templates It came into being.
Step Template (Step Stencil)
Due to the difference in the amount of solder paste required for soldering various components on the same PCB, the thickness of the same SMT template is required to be different, which results in the STEP-DOWN & STEP-UP process template.
Locally thin the template to reduce the amount of tin soldered to a particular component.
Bonding Template (Bonding Stencil)
The COB device has been fixed on the PCB, but the tin-printing process is still required, which requires the Bonding template.
The Bonding template is to add a small cover to the PCB bonding position corresponding to the template to avoid the flat printing of the COB device.
Nickel Plating Template (Ni.P. Stencil)
In order to reduce the friction between the solder paste and the hole wall, easy to demould and further improve the release effect of the solder paste, at the beginning of 2004, a special post-processing addition process was added on the basis of the traditional electro-polishing process. – “nickel plating” and patented. The nickel plated template combines the advantages of a laser stencil with an electroformed stencil.
Originally imported 301 steel plate, etched steel mesh is suitable for PCB printing with angular position and spacing greater than or equal to 0.4MM. It is suitable for copying and film use, can use CAD/CAM and exposure mode at the same time. It can be scaled according to different parts, which is not necessary to calculate the price according to the number of parts, and the production time is fast.
Stencil Effect on PCBA SMT Process
Stencil is a kind of mold dedicated to PCB chip processing. Its quality will directly affect the quality of printed circuit boards.
For example, thick stencil will cause a solder bridge short, too thin will cause an insufficient solder. Big stencil aperature size can cause a solder bridge short, if too small, the insufficient solder paste will be applied.
Poor stencil alignment will cause a solder bridge or an insufficient amount of solder after reflow. So, it is important to take a little extra time to ensure precise alignment between the PCB and the stencil.
In the process of SMT assembling, it is often necessary to solder the solder paste to the specified position of the electronic circuit board through the stencil. If the SMT stencil has a problem like not smooth opening wall and deviated the shape of the opening wall, it will cause the solder paste to stick to the opening wall during the processing of the patch, resulting in incompleteness. Welding the solder joints on the electronic circuit board makes the entire electronic board unstable.
In addition,it is best to use a circular-shaped stencil, its size should be slightly smaller than the PCB pad size, preventing a bridging defect during reflow. If the shape is deviated and does not meet the requirements for SMT processing, it will directly affect the processed product. Therefore, the quality of SMT stencil is very important for patch processing.
Last, don’t release target PCB from the machine prematurely. Otherwise, it will cause the solder paste shape to deform and may affect the next assembly process.
Stencil plays an important role in PCB and SMT PCB Assembly.
Therefore, as a PCB Assembly Manufacturer, in order to minimize the impact of SMT processing, it is necessary to select a reliable partner, pay attention to the use of stencil and late maintenance storage. This way can guarantee the quality of the stencil during the processing of the patch, thus ensuring the final PCB and PCBA product quality.
The picture is the PCBA that our company will produce for Australian customers, for swimming pool sterilization devices. This time the customer needs us to produce a sample of 10 pieces PCBA first. Looking closely at the figure below, we see the position number U1, which is a digital signal processor and controller chip, model dsPIC33EV256GM006-I/PT, with a pin pitch of only 0.5mm, in order not to affect the quality of SMT processing, our SMT engineer decided to make stencil in a high-quality and high-density company, only in this way, we can avoid the uneven opening window of the chip and the uneven tin leakage.
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