The LQS series introduced in "Anecdote of inductor development [No.3] The LQS series realized super tight tolerance by trimming" was a product with peak performance. LQS series products received high acclaim from analog circuit designers for the fully closed magnetic circuit, super-high Q, tight tolerance of ±2%, and above all else the incredible performance of a variation of just ±0.2% after reliability tests. This article describes the trends in LQH series electrode shapes and solder mounting technology in accordance with changes in the mounting environment.
Around the time that the first-generation LQN5N and second-generation LQH32M types were being produced, reflow mounting was used to mount components on some hybrid IC boards, but mounting was mainly performed by flow mounting to the rear surface of lead wire component boards. This mounting method consists of applying adhesive to the center of the board land or the center of the chip component electrode area, allowing the adhesive to harden, and then dipping in a flow solder vat. LQH series products are larger in size than CR chip components, so they are more susceptible to solder flow resistance and easily fall off. This was dealt with by creating a protrusion (called a stepped core) in the center of the electrode area. Thinking back now, it was a reverse stand-off shape, but of course it is no longer used.
As jet flow solder mounting became mainstream, components that are not subject to electrode corrosion and have high solder heat resistance came into demand. For this reason, electrodes changed from Ag and AgPd electrodes to Ni＋Sn plated electrodes, but this gave rise to a serious technical issue. Ni is a strong magnetic body, so magnetic flux concentrates on electrodes with an Ag thick-film＋Ni＋Sn plating layer structure, which greatly reduces the Q value. Here, in an example of reverse thinking, it was noticed that among coil applications, choke coil performance improves as loss increases--that is to say, as the Q value decreases. At the time it was not customary to sell coils separately by application, but as soon as we commercialized "dedicated LQH32C-type chip inductors for choke coils," we received orders from many customers, and it became a product that helped fuel the growth of the LQH series.
[New electrode forming method?]
Well, there was also the improvement of high Q-value products. This was quite a struggle. We spent long hours every day searching for a Ni alloy with both low magnetism and high specific resistance. The state of electroplating and electro-less plating technology at the time was such that it was difficult to simultaneously realize both a nonmagnetic, high-resistance material and thin-film control. During the course of these efforts, a monumental decision was made to adopt an electrode forming method that can be used not just with Ni alloys but with all metals, enables accurate thin-film control, has extremely high electrode bonding strength, and can also be applied to various electrode shapes. This required investment in expensive facilities used in semiconductor manufacturing, but the company executives made the wise decision to extend this electrode forming method to all LQH series products. As a result, the LQH series evolved into products that simultaneously feature excellent solder heat resistance and high Q-value performance.
This article introduced three anecdotes illustrating how the electrodes of LQH series chip inductors changed in accordance with past trends in solder mounting technology. Needless to say, Murata continues to track changes in the mounting environment and constantly improve the LQH series electrodes.
Written by: T.M., Ceramic Production Dept. 4, EMI Division