The Screen Printing Process - Challenges & considerations 101
With the demand for printed circuit board manufacturers to build smaller boards with increased capability, coupled with challenges facing SMT such as the shortage in surface mount devices, it is no surprise that everyone is feeling the pressure to ensure their quality is second to none. Decreasing aperture size to accommodate smaller, fine pitch parts components requires greater accuracy, and the high demand for some electronic components means it's critical that waste is kept at a minimum.
Focusing on some key challenges during the production of the printed circuit board, including the screen printing process, we explore how to overcome some key obstacles and how to introduce best practice into your processes to ensure you benefit from continual improvement.
Contact between the PCB and stencil
It sounds obvious, but it's surprising how many issues there are ensuring there is good contact between the PCB and the stencil. There are a few variables which can have an effect on the contact, including the aperture and shape, cleanliness and maintenance of equipment, as well a the thickness of the aluminium frames.
- Aperture variables - Some apertures have a narrow pitch and this can often lead to the solder paste spilling out of the aperture, causing a blur from one aperture to the next. Equally, in minute apertures, adhesion can often be problematic because the lack of solder doesn't reach the pad.
- Cleaning and maintenance - ensuring daily cleaning and maintenance is carried out can be critical to alignment. Over time, the stencil plate and the PCB can become misaligned where they are no longer parallel to one another.
- Aluminium stencil box frames - available in different wall thickness from 1.5mm to 3.5mm. It can be tempting to keep the costs down and purchase the thinner walled frames, because it impacts the price. However using the thinner frames can also affect contact because they can distort with use. Although purchasing thinner walled frames can be tempting, in reality it's a false economy.
It’s worth noting that ASM have designed a VectorGuard system, to help counterbalance the issues noted earlier. The DEK VectorGuard utilises a solid, high tension frame for fine pitch applications. Designed with compressed air, the VectorGuard clamping increases tension by up to 45%, providing a much greater transfer efficiency.
- Stencil vacuuming functionality - despite all of the potential variables that can affect the contact, there are functions that can help overcome some of the issues, such as stencil vacuuming. Even if the plate is showing signs of distortion, utilising the vacuuming functionality on the conveyor holds the stencil down, which stabilises the printing. It also helps to prevent any movement caused by the squeegee during the printing process.
Not all systems offer stencil vacuum functionality and if they do, it is often an optional extra. One of the solutions offering vacuum functionality as standard, is the Yamaha YCP10 Stencil Printer.
Filling the apertures with solder paste
Filling apertures with solder paste can be a challenge. Ensuring apertures are filled adequately i.e. not over filled or under filled, requires quite a few considerations. These can range from the quality of the solder paste, to factors such as the pressure, speed and angle the paste leaves the squeegee. Below are some of the considerations required to ensure apertures are adequately filled.
It’s easy to assume that following the data from the Gerber File should lead to a perfectly printed board. However, there are quite a few variables between the beginning and the end of the process, that can cause a range of imperfections to occur. Temperature and consistency of the paste, ambient temperature and humidity, squeegee pressure, and the separation speed can all affect the quality of the paste and overall alignment.
- Temperature - As paste is thixotropic and can therefore change its viscosity under stress (such as a change in temperature) it is essential that the temperature is kept constant, ensuring the viscosity remains stable. The viscosity of the paste will dictate the pressure and speed required to create the board. If this changes, and the pressure and speed is kept the same, some imperfections will occur such as misalignment issues.
- Pressure - It’s important to get the squeegee pressure right, to prevent stencil deformation patterns. Most pastes work well with a pressure of 0.45 – 0.57 kg/in force, but if you prefer to be cautious, then set the squeegee initially low and start testing until you get a clean wipe.
- Speed - As the squeegee speed decreases and slows, the printing volume gradually increases, improving the fill of apertures. However, reducing the attack angle can also have significant advantages. As the attack angle is key to the volume, reducing the angle to 65 degrees (depending on aperture size), can increase the volume of solder in the aperture.
However, adjusting the angle of attack isn’t always that easy to achieve. This is because there are only a few solutions that provide the capability for engineers to adjust the angle of attack. One of the solutions that allows such adjustment is the Yamaha YCP10 Stencil Printer.
Optimising adhesion through design
Ensuring the solder paste is touching the lands is critical in the design phase. Stencil aperture size should typically be reduced by 10%. This is particularly the case where apertures may prove problematic (size or shape). Etching conditions also need to be considered, and in some cases it might be wise to test incremental reduction in percentage until ideal adhesion occurs. If there isn’t enough land size in relation to the aperture, especially on minute apertures, then there will be a high probability that some detachment defects will occur. Other variables that need to be considered in the design process include:
Stencil thickness - The ratio of the stencil thickness (or lateral area) in relation to the bottom area, increases the chance of detachment occurring. Although there are ‘ideal’ ratios between the two area dimensions, the shape of the aperture can also affect this ratio value.
The shape of the apertures - The ‘ideal’ ratio for a circle, is no more than 1:7, and less than 2:0 for square. The corner of the squares can be problematic and so ensuring the ratios are correct and the speed is right, should minimise friction.
However, if these ratios are any larger, the stencil thickness should be re-evaluated. Most solder paste suppliers suggest - fast and constant is best to achieve good detachment.
Surface characteristics - Is another variable of the stencil that can have an affect on detachment. Both the texture of the aperture wall surface (rough or smooth), or its repellency can change the effectiveness of detachment. For example, the additive process tends to provide a smoother wall finish and lasered tends to leave a rough or textured finish. The smoother finish often provides better detachment, but using the additive process is usually more costly compared with laser, so it depends on what you are trying to achieve.
Although features of the aperture (such as the shape and surface characteristics) can cause issues with adhesion, there are features available with some systems that can encourage the solder into the aperture. The vibrating squeegee available with the E by DEK Stencil Printer is a feature that helps to manipulate the solder into the aperture. The Paste Height Monitor is also another feature of the E printer, which can ensure the optimum quantity of solder, by measuring the solder height utilising a calibrated laser.
This in turn will change your approach to everything discussed above. For this reason, all of the material you are working with and all of the tools you’re using needs to be considered holistically, to enable you to achieve the best results.
Taking everything above into consideration, we have listed the main considerations below, which can be utilised as a checklist. There is no winning formula, because every aspect from the design (including aperture size and shape) to the thickness of the aluminium frame needs to be considered. Some factors can be counterbalanced with tools and functions on your equipment, such as the stencil vacuuming function, helping you to achieve the best results.
1. Ensure cleaning and maintenance is carried out regularly
2. Invest in aluminium stencil frames with a good wall thickness
3. Utilise vacuum functionality to counterbalance any potential distortions
4. Test the speed, pressure and angle of the solder from the squeegee to obtain the right configuration for your design
5. Ensure the stencil aperture to land size is reduced by 10% as a starting point
6. The stencil thickness needs to be 1:7 ratio for circle apertures and less than 2:0 for square apertures
7. Consider the characteristics of the stencil including its texture and repellancy