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Surface Mount Components Show Their Hidden Benefits
December 31, 1969 |Estimated reading time: 3 minutes
By Craig Hunter, Vishay
Surface mount components usually are marketed with their most apparent benefits in view. Space and height savings compared to thru-hole devices often make up the key selling point. Automated assembly is simpler and faster compared to thru-hole device insertion, and typically surface mount devices cost less. But a look at some recently introduced surface mount components, both passive and semiconductor devices, shows some benefits enabled by this technology that are nearly as important without being quite so obvious.
For example, the current-sensing resistor shown in Figure 1 has an unusual pinout that is immediately noticeable. What might not be so readily apparent is that its copper-plated, welded terminations are flattened such that they make intimate contact with the resistive layer of the device along the entire side of the resistive element. The result is to minimize temperature variations. The resistive element is designed in a highly uniform way to dissipate power without creating hot spots, and the welded material is compatible with the element material.
These design factors result in a part with low thermal electromotive force (EMF), since the uniformity and thermal efficiency of the design minimizes the temperature differential across the resistor, thereby assuring equal and opposite thermal EMF generation at the leads. This further reduces the "battery effect" seen by most current-sensing resistors since the parasitic voltage generated at the junction of two dissimilar metals is minimized, while the pure current-to-voltage conversion is protected from such interference in DC applications.
The power semiconductor package shown in Figure 2 provides another example of surface mount design that delivers some valuable benefits over the long term, which may not be readily apparent. The main goal of this package is to dissipate heat efficiently and thus allow more power to be managed in less space. Instead of relying on just the leads for this purpose, internal parts that would be completely encased in a small-outline package are exposed, providing a more direct path to the PCB and ambient temperature.
Figure 2. Power semiconductor package.Not so obvious is the choice of which internal components are exposed and why. One approach, and the earliest approach taken with this type of thermally efficient package, is to expose the silicon die itself. This has the benefit of bringing to the surface, as it were, the hottest component of the device.
But there's a drawback. Advancements in the silicon technology used in power semiconductors usually result in a shrinkage of the die area needed to handle the same amount of power with the same efficiency. This shrinkage in die area is also what makes it possible for the cost of a certain level of performance to become less expensive over time. If the silicon die is exposed by the package, then this means every time the size or shape of that die changes, the pad layout on the PCB has to change as well.
For this reason, the power semiconductor package shown in Figure 2 does not expose the silicon die itself but the leadframe, which is attached to the die but doesn't change in shape or size from generation to generation of silicon. This means that a manufacturer of a notebook computer or server can design a power supply once and then evolve it without having to rearrange the board layout as new generations of silicon become available to make handling power more efficient or less expensive.
The world of components provides more examples of subtle but meaningful benefits provided by surface mount technology that are helping electronics manufacturers to lower costs. One final such example that is quite elegant in its simplicity is the LLP1006 package shown in Figure 3, a semiconductor ESD protection device. Note the similarity of its appearance to a 1005 varistor or SOD923, for which this device provides an active alternative that fits in the same footprint.
But there's another benefit that is exclusive to some versions of this package. Unlike the standard LLP1006, this package provides a clear view of the contour of the molten solder where it touches the component leads, which is crucial to verifying how well the solder has wet the metal. Typically this solder meniscus can be viewed only with X-ray equipment. By making it visible to the human eye, manufacturers can avoid the extra investment in time, equipment, and safety measures that X-ray inspection requires and thus significantly reduce their costs.
Craig Hunter is an SMT Editorial Advisory Board member and director, global Internet marketing at Vishay Intertechnology, Inc. Contact him at craig.hunter@vishay.com.