Fig 1. A cross section of a copper-core embedded module shows passive components embedded within the substrate, forming a module.
Fig 2. Because of their ability to closely embed decoupling capacitors beneath semiconductors, copper-core designs can keep ESL very low with little fluctuation.
Fig 3. Comparing copper-core noise performance to an FR-4 model, the copper-core design exhibits 5-dB to 10-dB better performance.
Fig 4. Based on the results of heat cycle testing, copper-core embedded modules provide reliable heat dissipation performance while retaining strength and rigidity.
In response to today’s market demand for full-featured handsets and smart phones, developers are continually adding functionality, such as music, video, cameras, and more. To cost-effectively incorporate this wide range of features, mobile device manufacturers have looked to modules as a solution for functional circuit blocks.
The problem, though, is that conventional mounting methods have been unable to meet the ultra-thin, space-efficient module-mounting requirements of manufacturers. New and innovative mounting solutions including copper cores are becoming available to overcome these limitations.
Trends In Mounting Methods
Low-temperature co-fired ceramic (LTCC) substrates are one of the latest considerations for use in mobile device applications. LTCC is a multi-layer glass ceramic substrate, co-fired with metal conductors. This process does achieve increased surface flatness. However, there are limitations in capacitance values.
One particularly promising solution is the embedding of components in organic printed-circuit boards (PCBs). This technique, first used in Japan in 2006 with applications limited to modules for cellular phones, offers high values of capacitance.
Embedded Substrate Component Solution
As interest in embedded substrate components increases, surface-mount and electronic components manufacturers are employing their technologies to develop solutions. Some of these manufacturers are using flip-chip technology on FR-4 circuit board material.
Other manufacturers have developed substrates with copper cores in which ICs and passive components can be embedded (Fig. 1). This technique outperforms FR-4 versions and results in a wide range of benefits. One example of copper core substrate use is embedded organic module involved nanotechnology (EOMIN) from Taiyo Yuden.
By using a copper core, designers can achieve one of the primary goals of mobile device design—a thinner profile. The copper serves as the core material, wiring board, and ground layer in these embedded designs, enabling the production of modules that are only 0.48 mm thick. Embedded designs with a copper core also allow for outstanding noise resistance, heat dissipation, and high rigidity/reliability characteristics.
Speaking of noise resistance, in high-speed semiconductors, circuits must be designed with low equivalent series inductance (ESL) by placing decoupling capacitors in close proximity within the circuit. With copper-core embedded designs, circuits easily achieve low ESL, as decoupling capacitors can be embedded closely beneath the IC, creating the shortest possible wiring lines. Additionally, since the core acts as a shield from the magnetic flux of the capacitors, its ESL is lowered (Fig. 2).
The characteristics of copper-core embedded module radiant noise from distant electromagnetic fields can be compared to an FR-4 model. In Figure 3, the blue shaded region indicates noise from vertically polarized waves, the light blue broken line above that indicates noise from the FR-4 model, and the blue line indicates the copper-core embedded model.
The pink shaded region indicates the noise from horizontally polarized waves, the pink broken line above that indicates the FR-4 model, and the red line indicates the copper-core embedded model. Furthermore, copper-core embedded designs improve the performance of horizontally and vertically polarized waves by 5 to 10 dB over other models.
Better heat dissipation is another copper-core advantage. Compared to an FR-4 module with embedded components, copper-core embedded modules provide better heat dissipation performance by conducting the heat generated by ICs through its copper core and pad to the motherboard.
Rigidity And Reliability
The use of electro-copper plating to connect embedded components eliminates de-wetting found in conventional soldering methods, avoiding heat expansion stress on melting solder paste. Test results verify that electro-copper plating connects demonstrate excellent heat-cycle resistance with modules capable of withstanding up to 1000 heat cycles (Fig. 4).
Copper-core embedded modules feature increased strength and rigidity. In an example where a semiconductor was mounted on a copper-core embedded module while the motherboard was deformed, the stress to the semiconductor was minimized and excellent drop impact resistance results were achieved.
The unique characteristics of copper-core embedded modules make them especially well suited for reducing the size of functional modules, enabling space savings in a range of wireless handsets and mobile devices. Taiyo Yuden’s EOMIN multilayer organic (resin) substrate features a copper core in which ICs and passive components may be embedded.
As components are mounted on them, they become EOMIN modules. Electro-copper plating is adopted to connect the embedded passive components electrically, which has the advantage of eliminating the de-wetting in soldering that may occur in conventional soldering.
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