Specialist answers from professionals
to your technical questions
By and by we will provide an overview of the most common questions posed by our customers on topics such as the optimisation of SMD stencils, solder paste printing, quick-tensioning systems, and BECKTRONIC products and services:
What are the optimal contact/landing surfaces for chip designs?
The optimal position of the component contact surface to the printed circuit board landing surface and their paste application has a significant impact on the soldering outcome of SMD components. This also applies to supposedly simple chip components. It is not uncommon for amplified soldering problems to occur here, because the size ratio or the position was not coordinated.
Optimally, the landing area should be just below the contact surface of the component. The surface should be slightly larger to the outside here to present a clean meniscus. If possible, the LP surfaces should not protrude significantly inward under the component body. The recommended values depend on the exact design or size. The 0603 design is given as an example here. Its dimensions are approx. 1.6 mm x 0.8 mm.
The appropriate landing surfaces have a size of 0.5 mm x 0.9 mm, whereby an internal distance of approximately 0.9 mm is recommended. These values promise optimal soldering results with precisely adapted paste application. Due to various factors, it is not always possible to adhere to these values. A correspondingly optimised paste application can nonetheless be achieved. At BECKTRONIC, we support you with advice and service.
How can a stable, reproducible lowering and contacting of QFN components be achieved?
The right paste volume is essential for an optimal soldering result. The volume connection of the "ground plane" of the exposed pad is particularly crucial. In principle, it is advantageous if the paste application of this surface exceeds the total area of the surrounding contacts by a maximum of 100%. The distribution (screening) of the paste application also plays a major role. It supports or favours the venting of flux residues. Any existing vias within the exposed pad area must also be taken in to account. Care should be taken here that no paste leaks uncontrollably to the underside during the application of paste as well as during the reflow process.
The paste application of the contact pins depends on their exact position and the condition of the edges (wetted or not wetted), and the paste is applied accordingly.
How must the stencil be layouted to correctly assemble a mix of components from 0201 to 2010 and right through to THT components?
Nowadays, it is increasingly common for a printed circuit board to consist of a wide variety of designs. Large components or structures with a high heat capacity located in the vicinity of very small designs can lead to soldering problems (thermal shading). In general, care should be taken that each design receives its own optimum paste volume. An adapted stencil layout can be used to compensate for individual mix structures.
However, if the complexity and difference in the required paste volume is too great, a stencil in step design is the tool of choice. This allows different material thicknesses to be achieved within a stencil. Each individual area can then be optimally tailored to the needs of the assembly. Since each layout is assembled differently, BECKTRONIC always advises individually and in line with customer requirements. The following decisive parameters are analysed and taken into account among other things: the component geometry, the solder stopping area, the component connection, the soldering method, the paste type, and the squeegee blade direction.
How can tombstoning be avoided?
The paste volume should be adapted to the chip design and optimised if necessary. Effective options here are surface and design adjustments. The homeplate and the bow tie have proved themselves especially useful in the past. The "bow tie" design in particular is being increasingly used. Additional attention must be paid to the position of the paste application. If a pressure offset occurs between the landing surface and the paste application, the exact positioning on the target position leads to an increased risk of tombstoning.
How can I reduce air pockets (voids)?
Voids (cavities) occur when the flux has to travel a long way to the outer edge of a component surface (up to the pasting edge). In order to ensure a better, more homogeneous removal of the flux, areas (from approx. 6-8 mm²) are usually rasterised.
- Solder paste:
Additionally, solder pastes with a lower proportion of flux tend to be less prone to the formation of voids.
- Soldering process:
The optimisation of the soldering process with the aid of different air pressure variants (positive and negative pressure) also significantly influences void formation. This is an example of how an improved soldering result is achieved with reflow or wave soldering when nitrogen is added. The nitrogen serves as a protective gas, which reduces the oxidation of the solder and the components by oxygen. In addition, the surface tension of the liquid solder is positively influenced.
How can I prevent the "jettisoning" (popcorn effect) of components?
The reason for these are sealed cavities under components (heat dissipation in the exposed pad of a QFN) or within large paste surfaces, as well as "plugged vias", which have not been properly closed. Air pockets lead to an explosive process at this point. If the vias are not completely closed in accordance with IPC 4761 Type VI, the trapped air cannot escape and excess pressure develops under the component body. In order to counteract this process, the paste should preferably not flow into or close these sleeves during the melting process. In general, the moisture content of SMD components and printed circuit boards should also be taken into account. Otherwise, this may lead to unwanted delamination (of the LP surface) or breaking up of SMD components. An upstream annealing process (baking) can be used to remedy this.
How can I avoid the twisting or tilting of components?
Even at this point, adapting the surface paste application can be the solution. With cylindrical designs such as MELF or micro-MELF, a concave inner surface design or U-shaped paste application can guarantee secure positioning even after the soldering process. In QFN designs, reducing and rasterising the exposed pad helps to fix and evenly lower the pad.
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