There are two known methods by which electronic components are attached or mounted to the printed circuit boards---spanThrough-Hole Technology(THT) and Surface Mount Technology(SMT). THT is a method of PCB assembly wherein component leads are attached to the PCB by inserting them into the drilled holes of the boards. SMT, on the other hand, utilizes solder paste technology to mount the passive and active components into the PCB.
SMT is the more recent technology that is widely used today and is sometimes used in conjunction with THT. In SMT, a PCB stencil is used during the solder printing process to be able to effectively distribute the paste into the contact pads of the boards. More details about PCB stencils are discussed in the following sections.
A PCB stencil is an important tool in surface mount technology (SMT) to be able to deposit a correct amount of solder paste into the printed circuit boards at accurate positions. It is a thick and flat metal that consists of apertures that are equivalent to the solder pads on the boards. A stencil contains fiducial marks which serve as alignment references during processing. Stencil printing is the most practical approach to achieving a fast solder deposition for mass production use.
The circuit board stencil printing process is the first major step in SMT which is often used in the single-sided PCB assembly and double-sided PCB assembly. A successful SMT process should have optimisation studies of the following key variables, such as stencil thickness and apertures, solder pastes condition and metal filler sizes, squeegee speed, pressure and angle., etc. Here we talked about the solder paste printing process in the use of PCB stencils.
Firstly, we need to put the PCB on the paste printing machine. The stencil for PCB assembly will be fastened and clamped to avoid movement. Both automated and manual printing machines can be used.
When the PCB stencil is ready, it is time to add a proper amount of solder paste to the circuit board stencil. But make sure the paste is mixed properly in advance since solder paste needs to be stored at 4°C to 10°C.
The PCB board is clamped and aligned with the circuit board stencil such that the apertures coincide with the contact pads where the solder is to be deposited.
The squeegee then moves on the circuit board stencil to evenly distribute the paste into the PCB stencil apertures. The solder paste will go through the apertures and finally adhere to the PCB pads.
The circuit board stencil is separated from the circuit board then the circuit board gets unclamped and moves to the next process. For fine-pitched solder pads, it is recommended that the separation is done in a straight-motion manner to make a precise printed solder paste.
Post-solder printing optical inspections are carried out to verify the correct paste volume and alignment. A built-in looking-down camera is used to automatically inspect the deposited solder paste. The solder paste inspection process can be done by the solder paste printer machine if it has the feature or by a separate inspection machine.
A mandatory step to ensure consistency of solder printing quality is to clean the stencil. Cleaning is important to remove solder paste residues that adhere to the aperture walls. This must be done at regular intervals to avoid paste deposits from hardening and affecting the next process. It is even more critical for finer components. Stencil cleaning can be done manually or automatically. Modern circuit board stencil printers have spray tanks that can be used to clean the stencil in PCB after printing.
Below are the different considerations and techniques in designing stencils for PCB assembly manufacturing.
Circuit PCB stencils can come in various shapes and sizes in single-sided and double-sided PCB assembly. The most common shapes are circular and rectangular apertures. Other aperture shapes can also be designed to support the shape of the components to be mounted. They must be designed with the right length width and thickness with consideration to the solder paste type, solder pad size and component pitching. The thickness influences the volume of the solder paste. Fine-pitched PCBs may need to have thinner stencils to lessen solder overflow and bridging. Excessive thickness may cause the paste to adhere to aperture walls due to surface tension.
Some circuit board stencil apertures are intentionally designed to have reduced aperture area compared with the Gerber design allowing for solder paste spreading during the collapse of the paste during the reflow process. IPC standards describe two important dimensional parameters in PCB stencil design—area ratio and aspect ratio. Area ratio is the ratio of the area of the PCB pad and the area of the aperture walls while aspect ratio considers the aperture width and the stencil thickness. The dimensions of the circuit board stencil are also influenced by the solder ball size or the metal filler size of the solder paste.
In actual PCB assembly, particularly double-sided PCB assembly, the most common type of stencil in PCB is made from stainless steel due to its corrosion resistance, durability, and dimensional accuracy. Stainless steel does not easily deform with repetitive use and is best for high-volume PCB assembly. Polymer PCB stencils are sometimes used which are flexible types of materials made from Kapton. They are less superior and not very appropriate for high-volume PCB assembly.
For more complex PCBs, multi-step stencils or stepped stencils can also be designed. These are the types of circuit PCB stencils that have different thicknesses in a single aperture. This is typically applied to fine-pitched components wherein it is necessary to create a locally decreased area in the opening of the stencil design PCB. Multi-level stencils are usually manufactured by an initial chemical etching process and a subsequent laser-etching.
Stencils for PCB assembly manufacturing are made by referencing the PCB’s design layout through Gerber file data. Each stencil manufacturing technique results in a different quality of aperture wall. The quality is dictated by the roughness and precision of the stencil apertures. Below are the different stencil manufacturing techniques employed in the industry.
Laser cutting is best suited to form accurate apertures with neat edges. It is most applicable to fine-pitched and high-density designs. Laser cutting produces trapezoidal apertures for easier solder paste release. So laser cutting PCB stencils are widely used.
Chemical etching uses a subtractive method to form the pattern on the stencil. The metal surface is first cleaned, and a photoresist is covered on the surface. The photoresist is then developed and rinsed off using a chemical. An etchant spray Is used to etch away the unwanted metal.
Contrary to the other techniques, electroforming is an additive method using nickel. Electroforming is sometimes used in combination with laser cutting for more precise PCB stencil apertures.
Solder Paste Printing Issues And Solutions
Listed below are what potentially can go wrong with poor designs of circuit board stencils, and also the solutions from our experience.
| Issues | Reasons | Solutions |
|---|---|---|
| Misaligned Printing | Inaccurate circuit board to PCB stencil alignment may result in misaligned printing. | The stencil is 100% accurate, and also alignment should be done properly. |
| Solder Bridging | Incorrect aperture geometry can result in solder overflow and bridging during printing and reflow processes. | Make sure the apertures are correct and check the stencil with PCB before paste printing. |
| Incomplete Solder Coverage | Too narrow stencil openings will result in insufficient solder volume and incomplete solder coverage. | Experienced engineers to general proper stencil files. And compare the stencil with PCB pads when finished. |
| Solder Balls | Besides reflow soldering issues, the reason for the PCB stencil side can be stencil thickness and aperture sizes. | Appropriate stencil thickness and aperture are used, and anti-solder ball stencil should be used. |
If you need to use surface mount technology, a reliable PCB stencil manufacturer is a must-be. High-precision stencil contributes to surface mount accuracy. USPCBA as a reliable PCB stencil manufacturer. Here are some of our advantages
1. Being Familiar with PCB Stencil of various PCBs can give you a satisfactory solution, especially stencil for PCB assembly.
2. We ensure the whole process of high-quality PCB Stencil, to provide you with top assembly of Stencil PCB.
3. We can provide customised PCB stencil production services according to your needs and can deliver on time
4. Support small and medium-sized batch production of single and stencil for PCB assembly manufacturing, tailored for you
5. Effective communication and technical support throughout the stencil PCB assembly manufacturing process
We have a professional team and production workshop to provide you with first-class PCB Stencil customisation services for stencil PCB assembly manufacturing. To get your ideal stencil PCB, please contact us and place an order.
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