SMD Coils – Yes, No or Maybe?

 

Old hands are not necessarily warm towards the idea of surface-mount in general; surface-mount inductors are considered the devil’s handiwork. On the other hand, integration and assembly cost pressures motivate even the most conservative designer to take a second look at SMD.

Playing around in your workshop is likely to make you paranoid; in addition to “inductor levitation” issues which laid flat many Palm m5xx organizers, it takes but a cursory look at the frontend of the venerable HP 34401A to see impossible-to-find Gowanda inductors going AWOL and causing the dreaded Error 612, 613, 615, 617, 618, 619, 621 combination.

Read a book…

Hollomon’s classic “Surface Mount Technology for PC Boards” contains a surprising, albeit logical statement:

“in principle, SMD components are more vibration robust than their THT compatriots due to their lower weight.”

This is easily explained – keep in mind that the force F is a function of mass multiplied with acceleration. Reducing weight due to missing leads, thus, leads to better attachment of the component to the board during vibration tests.

Before diving into specific interesting products, let us mention that this rule does NOT apply to connectors. With them, force is applied by the user and is not a function of weight – the larger area of the solder joint can be beneficial.

Inductors come in a large variety of types. One product, which we found to get a lot of attention recently, was a rather simple drop-in replacement for a classic coil. Simply deploy it to get rid of two or four through-hole joints, thereby reducing assembly costs.

 
 

This, of course, is not all you can do. Switching legacy switching ICs for high-frequency ones such as the ACT4065 we introduced recently allows you to reduce inductor size, thereby killing both weight and cost.

Advanced coils

Just like in classic inductors, a large variety of construction styles exists. Wurth’s WE-PD family, for example, has a shielded core which reduces inductor emissions – this is helpful if the power supply is physically close to sensitive circuitry like a GPS receiver, but can not be shielded efficiently.

 

Bourns provides a very interesting application note which goes into more detail on the various inductor types – it is a well-recommended reading material for all those who are not 100% firm in inductor design and behavior.

In Conclusion

Electrical engineering and electronics is both art and science. However, “tribe mentality” can be a valid guide – given that tons and tons of SMD coils are sold every month, things can not be all that bad. Of course, we look forward to your feedback.