SMT solder paste printing technology guide (below)

Separate printing

High density stencil printing

Interstitial printing is used in high-density stencil printing or screen printing. The gap value setting refers to the distance from the template/web to the surface of the printed substrate before the doctor blade contacts the template/screen in the standby state. This process allows the stencil or screen to be rolled through the substrate and stripped while producing a consistent solder release rate. For high-density printing, if contact printing is used, the separation of the stencil or the screen will be different at the edges and center due to the influence of adhesion.

Template design

In some cases, when an operator wants to place two graphics on a template, the question arises: How far apart should the graphics be?

Normally, for the drag blade squeegee printing, the pattern should be separated by at least 3 inches. If a slight overprint distance adjustment is also allowed, the distance should be increased to 4 inches (100 mm). For stencils that are 29 inches by 29 inches in size, the maximum width of the substrate is limited to no more than 6 inches (150 mm). When printing with MPM's Rheometric extrusion head, the spacing between the figures can be reduced to 0.75 in. (19 mm) and the overall width of the plate will increase to 8.9 in. (226 mm).

For efficient printing results, the minimum steel sheet size required varies. When using a drag blade, the minimum size of the steel in the Y direction should be the size of the board plus 7 inches (178 mm). When using a diamond spatula, the minimum size of the steel plate in the Y direction should be the size of the plate plus 1 inch (25 mm). In the X direction, the minimum size of the steel plate should be 1 inch (25 mm) longer than the length of the blade.

It is recommended to center the graphics of the board on the template to achieve true vertical separation of the template from the substrate. If the pattern is not in the center of the stencil, there will be some non-uniform delamination when the separation occurs due to the adhesion of the flux oozing out between the bottom of the stencil and the substrate.

For the correct template and substrate alignment, positioning the datum point is always the first choice to get the best alignment results. If the traditional positioning reference point is not available, using the pad on the board and the opening of the template as a reference point can also provide acceptable results. A unique area on the board should be used. For example, the corresponding position on the last pad of the QFP on the substrate can be used for alignment. Care must be taken to keep the opening of the template clean, as clogged openings can cause misalignment. That's because the size and shape of the clean openings during programming will be different from the clogged openings in production.

Example 1 (a good example of setting using a QFP device)
As shown above, this creates a unique graph, because the system will also search for blank parts to the left of the target pad, but there is no other target that can fit in the camera's field of view.

Example 2 (a poor example of setting up using a DIP device)
The problem with this setup is that the search area is not created in a unique area, so there is a possibility that the system will find a mismatched target, which causes the system to align to the pad at the bottom of the field of view instead of the desired top pad .

2D inspection

One frequently asked question is why 2D verification is required? A 2D system is designed to be used as a process validation tool. They are not inspection systems and cannot be used as an alternative to inspection systems. In short, 2D inspection helps you to develop your process and verify that your process is within control. If your process never changes, you may not need an inspection or verification system. However, most printed circuit boards have challenging devices or applications, especially fine-pitch devices that require some form of verification.

The 2D system is designed to help you verify that the release rate and coverage of the solder paste meet the pre-established benchmarks. The 2D verification system will help and warn you about any deviations from established processes, allowing you to make the necessary adjustments before additional additional process costs (such as cleaning the substrate or repairing) occur.
An interesting statistic you should realize is that 90% or more of print defects and trends can be identified by examining the amount of solder paste that covers the target pad. This method can identify the lack of solder paste accumulation on the pad, which can lead to unacceptable solder joints. Most effective 2D inspection forms incorporate a grayscale comparison technique to determine the percentage of coverage of the solder paste over the target PCB pad. This technique compares the uncovered area of ​​the printed pad and the area of ​​a saved empty pad, which is determined when the device is first programmed. With this data, the following calculations can be used to determine the percentage of solder paste coverage:

Parallel processing capability

The functional characteristics of all devices are designed to work effectively. However, in practice, all of these functions are performed in a serial manner. In other words, when the device is performing its auxiliary functions, the device does not perform its basic functions. More precisely, the basic function is to print solder paste on the PCB. Of course, the auxiliary features of these devices are needed for the goal of completing as many quality products as possible each day, especially for complex boards or in high-volume production environments. Because we have lost the actual production time to the required but “auxiliary” function, we are still dissatisfied despite the fastest cycle time of the equipment.

Ideally, we want our process equipment to perform some (if not all) necessary auxiliary functions while performing (concurrently) basic machine functions, or performing one or more auxiliary functions at the same time.

Devices that can run the necessary auxiliary functions in parallel will optimize the production cycle and yield. For example, if the solder paste printing device can move the next product while cleaning the screen and perform the visual alignment function, then we have two auxiliary functions to be executed at the same time, and the result will be to optimize the production cycle and quality. .

Using equipment that provides parallel processing capabilities can yield many advantages in terms of output. Providing quality-enhancing auxiliary functions, such as wiping screens and post-print inspections, can be performed quickly, and there will be no reduction in the output of printed circuit boards at the same given time. Parallel processing capability is a "good" situation, producing high-quality boards without the loss of time.

As the electronics industry continues to switch to smaller and more complex devices (0201, 01005, CGA, flip chips, etc.) and to convert to lead-free materials, in order to complete the company's financial goals, “start things right” Requirements are becoming more and more critical. For new products designed for these new devices using lead-free materials, the ability to rework will be a major challenge. At the very least, the rework of lead-free products will take more time and there is the possibility of damage to more devices and boards. Therefore, the best cost-effective solution is to start things right. We must continuously evaluate the characteristics of the equipment and equipment in order to maximize the likelihood of success.

Finally, suitable equipment and well-trained and experienced staff can effectively face the challenges of mold/plate and screen printing. As well as the waste of solder paste in the printing process, reducing the product switching and the intervention of operators during the new product setting process are all requirements for achieving high quality printing results. Of course, careful consideration of all printing issues helps manufacturers produce high-quality printed circuit boards and improve the entire printed circuit board assembly process

Source: China Electronics Manufacturing Network