The advantages of stencil printers with 3D inspection
Why we rely on 3D inspection for stencil printing in EMS production
Sven Olle is head of department in our SMD production. Among other things, he is in charge of assembly and plays a key role in quality assurance in this department. A large part of the quality is reflected in our very low error rates. In this article, Sven Olle explains why 3D inspections of the solder paste are so important.
The basics: What do stencils have to do with SMD assembly?
Before we can assemble, we have to apply tin to the mounting surfaces of the SMD components for the subsequent soldering process. In SMD assembly, this means that the solder paste must be applied (precisely) to the areas on the PCB where we want to place the SMD components later. And (exactly) in the right quantity for the component. This is done in the stencil printer, which basically works in the same way as screen printing:
- Using a stencil made from a thin (approx. 80-150µm) stainless steel sheet, we apply the solder paste to the as yet unpopulated PCBs.
- We have to have the stencil specially made for each newly developed, commissioned assembly.
- During the printing process, solder paste is applied to the PCB wherever the stencil has cut-outs.
Both the PCBs and the stencils have so-called read marks. These are used to precisely align the stencil and PCB with each other. Thanks to its programming, the stencil printer knows the distance at which these reading marks should be positioned in relation to each other. Any misalignment can be automatically compensated for by the printer.
Depending on the thickness of the stencil, more (thick stencil) or less solder paste (thin stencil) is applied to the PCB. The correct amount depends on the size and properties of the component to be assembled.
The layout of the assembly is crucial to the success of the project
When it comes to development, we support and advise our customers - and occasionally have to raise a few objections to the layout of the printed circuit boards. In some cases, components that need a lot of solder and components that need little solder are too close together. This is problematic, as the different solder requirements result in either excessively lean solder joints or short circuits. For these cases, there are step stencils with different thicknesses, but we can't compensate for everything. In such cases, we go into consultation or have a few little tricks, such as how we can apply more solder where necessary by making larger cut-outs in the stencil.
Stencil printer or jet printer?
If we stick with the metaphors: If the stencil printer works like screen printing in terms of assembly, the jet printer is the inkjet printer. At A+B Electronic, we use both. Because the jet printer does not require a stencil, it is more cost-efficient for small series, samples and prototypes, while the stencil printer can print a large number of PCBs at a much higher speed. This is also crucial for developers: they can rely on us for fast prototype production with a jet printer and get large quantities with a stencil printer.
Recognizes errors before they occur: Why 3D SPI is important for our customers
Solder Paste Inspection (SPI) is available in 2D and 3D. We rely on 3D for our high quality assurance requirements. With this 3D SPI, the printer measures the solder joints using laser triangulation. The 3D SPI checks three points, X, Y, Z, before the actual assembly begins:
- X and Y is the alignment of the PCB to the template and therefore the exact positioning of the solder joints. This works in the same way as on a surface on which a coordinate system is placed.
- Z is the height of the solder joint, i.e. the mass of the solder tin at a point on the PCB.
If the 3D SPI detects an error, the stencil printer interrupts production. It then takes a 2D image so that our experts can analyze the error. This means that an incorrectly printed PCB does not even make it to the assembly stage. In this way, the quality is also consistent - we minimize the margin for error.
3D SPI is an important element of our customers' success in series production. But we do even more: in addition to SMD assembly and THT assembly, we use selective conformal coating to provide our customers with an optimum coating and therefore long-lasting assemblies.
