The New Proportional Land Dimensioning Concept
Bibliothek des Wissens Band 18 (Englisch) - Library of Knowledge Volume 18:
In addition to reliable components and circuit boards, reliable electronics require equally reliable production. Following assembly with a low placement tolerance, a flawless soldering process of the components is particularly important. This in turn requires that the lands on the circuit board
match the component terminals exactly.
In the early days of SMD soldering technology it was assumed that for a reliable solder connection the soldering surfaces on the PCB should be
significantly larger than the component terminals themselves. This is still reflected today in the applicable standards of IPC-7351 and IEC 61191-2. Although repeatedly revised and adapted to the ever finer structures, the currently defined specifications and calculation methods for creating the
soldering surfaces fit less and less to the constantly changing component terminals, especially the smaller they become, above all because constant
absolute values are specified for the protrusions. For modern reflow soldering processes, these are outdated rules and measures, as practice has shown: Soldering surfaces that protrude too far generate lateral tensile forces, which in extreme cases can lead to the dreaded tombstoning (tombstone effect) in assemblies with chip components, for example, or solder bridges in multi-terminal components. So, too much solder and area can have negative effects during the soldering process. In addition, any unnecessary use of space is a considerable disadvantage, especially for densely populated assemblies with a high connection density. The constantly expanding, almost incalculable variety of component terminal
shapes, sizes and grid dimensions requires an adaptive dimensioning of the corresponding soldering surfaces for optimum solder joints. This is where the New Proportional Land Dimensioning Concept comes in: It enables the calculation of the suitable land on the circuit board in proportion, to the size of the component terminal, taking into account the physical properties of the soldering process: wetting, capillary action and force transmission. In their interaction, however, they cause the solder meniscus and coverings specified in the applicable standards under reliability aspects without unnecessarily or negatively affecting too large land areas. The starting point for the proportional concept are the requirements at solder joints as described in J-STD-001 and IEC 61191-2 on the one hand and a new simplified classification of components according to their terminal types on the other. While the IEC land pattern standards as well as the IPC-7351 differentiate the components according to package families, the proportional concept is based on a two-stage classification according to terminal types. The advantages of the New Proportional Land Dimensioning Concept are:
Easy scalable, Trouble-free adaptation also for future components, Risk minimization (e.g. bridging in paste printing, best adapted, paste volumes), Higher reliability of the solder joints (error-reduced assembly process), Proportional land pattern lead to more space for the routing process.
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