Modern components such as the EVIYOS LED matrix enable the precise adjustment of an automotive front light in real time. In addition to a suitable stencil layout, a constant printing process is also required for perfect function in order to achieve permanently reproducible results.
Advertorial | published in: EPP 10/2019
Author: Harald Grumm
Modern components such as the LED matrix EVIYOS make the precise adaptation of automotive front lights possible in real time. The DFN (Dual Flat No lead) package stands out on account of its extremely small space requirements. In order to work perfectly, these designs require optimum solder spot execution and thus precise planning of the stencil printing process.
Information about the product:
On the underneath, the component has five types of pads that differ in size. The objective of stencile print planning is to provide the right solder volume for each type of pad and to produce the same solder connection height respectively every time. This results in the component being parallel to the substrate surface after completion of the soldering process without producing any faulty spots or bridges between the pads.
The type of solder connection must also be taken into consideration. The dimensions and the relative
positions of the pad areas influence this.
The pad shape P1 is identical on the component and on the substrate, resulting in a rectangular solder connection with constant height. In this case, the volume is formed from the edge lengths and the solder connection height to be expected.
With pad shapes P2 and P3, the substrate pad projects from underneath the component. Here, application of the equation for a truncated pyramid makes sense for the minimum volume. However, it must be noted that an increase in solder on the end can absorb additional solder volume.
For thermal pads, the truncated pyramid equation is used by approximation in order to calculate the solder volume.
The same solder connection height (h) must be used for all three calculations. IPC-7093 recommends heights of (50 … 75) µm here, which are difficult to achieve in practice. Typical solder connection heights are between 25 µm and 40 µm. It must be mentioned, however, that reliability increases with the height of solder connection in the case of BTCs (Bottom Termination Components).
The solder volumes calculated are divided by the percentage volume share of solder powder in the solder paste and result in the solder paste volume. This is divided on the respective substrate pad in such a way that good release behaviour from the stencil is to be expected, as well as a low tendency to form bridges. In this case, the equation for area ratio helps:
as does implementation of the opening width with half the pad grid. This guarantees the maximum possible spacing between the stencil openings. Stencil layouts prepared according to these rules are good starting points for testing in practice. Further adaptation may be made necessary by outgassing, deviations in pad size etc.
In addition to the stencil layout, a constant printing process is necessary to achieve reproducible results. Suitable equipment technology must be chosen, which achieves high process stability independently of the operator and permits inspection of the printed solder paste volume directly in the printer if necessary. The VERSAPRINT 2 fulfils all these criteria. Its clear menu structure makes it easy to operate, and the camera systems available allow the printed solder paste to be inspected either two-dimensionally as an area or three-dimensionally as volume. Selection of the type of inspection depends on the assembly requirements. 3D inspection is recommended especially for small openings (designs <0402 or µBGAs). With these designs, deviations in the printing height caused by solder paste sticking in the stencil opening are detected – even if the basic area has been completely printed.
Additional options further protect the printing process, e.g. monitoring of the solder paste quantity on the stencil during printing or a camera-assisted support pin positioning for perfect support of the PCB even with complex substrates.
About the lecture
The lecture uses an innovative matrix LED design to demonstrate how the theoretical solder volume is calculated for the individual pad variants. The definition of the general conditions for an optimum solder connection design and the suitable method of calculation are explained. In addition, possibilities for inspecting the printed solder paste volume directly in the printer in order to guarantee the determined values in practice are also presented.
Harald Grumm studied precision and electronic device engineering at the Berlin University of Applied Sciences. He has been working in process engineering for SMD technology since 1996, focussing on stencil and screen printing. Within the scope of his work, he carries out training for stencil printing and is involved in different publications in the fields of stencil printing and module assembly.
He has been back at Ersa GmbH since 2016 and is now redefining the stencil printer series VERSAPRINT.
As the largest manufacturer of soldering systems, Ersa guarantees perfect connections in electronics manufacturing around the world and is responsible for smooth processes in electronics production as a system supplier and automation specialist. Founded around 100 years ago, Ersa is not only extremely tradition-conscious but also makes its claim to innovation and technology leadership quite clearly in the company vision.
Ersa engineers always focus on designing products and solutions in such a way that they optimise the production processes of Ersa customers. They thus set benchmarks with innovations, groundbreaking technologies and highly efficient production systems which make adaptation to the fast-moving changes in connection technology possible.
Within the soldering machines division, the portfolio includes solder paste printers, reflow ovens, vacuum soldering systems as well as wave and selective soldering systems and intelligent automation solutions. The assortment of manual soldering equipment ranges from the tried-and-trusted soldering iron through intelligent soldering stations to soldering robots and fully automatic repair stations for soldering and desoldering.
A wide range of services such as the Kurtz Ersa Know-How Transfer Programme and numerous application centres around the globe offer real added values for the customer.
New components are under high innovation and price pressure. This results in the use of low-price manufacturing technologies which usually do without complex connection designs such as gull wing or solder balls. It is here in particular that the BTCs (Bottom Termination Components) can win significant market shares.
This makes it all the more important to be able to reliably process these components in production. A theoretical preliminary consideration of the opening layout in the stencil is necessary for this. Unfortunately, there is often a lack of detailed knowledge in the layout departments of many companies, which lectures such as this aim to compensate.
In terms of printing, BTCs are generally easy to implement if grid dimensions do not fall below 0.5 mm and the substrates (PCBs) are made to a good quality standard. This means the installation stations should be level and the pad sizes observed. These there is nothing left in the way of a successful process with a good stencil printer such as one from the VERSAPRINT series.
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