How mSAP Enhances HDI PCB Capabilities

With 5G technology around the corner, we are looking at the emergence of 5G smartphones. While this requires new manufacturing technologies such as high-density interconnect Printed Circuit Boards (HDI PCB), smartphones need to be less expensive and produced at greater efficiencies.

Customers usually covet compact sleek devices. Therefore, manufacturers need to balance function and form so that their products stand out in a crowd in a competitive marketplace. The smartphone market can be a treacherous place with corporate fortunes rising and falling on the success and failures of specific generations of phones.

Smartphone designers tend to use every millimeter of space within the device enclosure to unlock significant value for the user. This is how they are able to fit in large and high-resolution displays, large batteries, and more sophisticated processors. This allows designers offer more functionality with an enhanced feature set, ultimately improving the overall user experience.

As most of the design of a smartphone is form-factor driven, PCBs in the form of high density interconnects are the major contributors. These HDI PCBs are specially designed circuits differing from conventional PCBs as they provide the designer with more functions per unit area. Their main advantage is finer copper traces, thinner and more flexible base material and laser drilled via holes. Although HDI PCBs have played a crucial role in creating miniature smartphones and other embedded subsystems, 5G technology demands are more severe.

The new generation smartphones compatible to 5G requires extremely complex RF front ends and antenna configurations involving multiple inputs and multiple outputs—generally known as massive-MIMO. This not only expands the footprint of the RF content within the phone, but also enhances the processing power necessary to control the staggering volume of 5G data. Simultaneously, all the extra features and functionality affects the battery capacity, and hence, the geometry of the phone. Conversely, if the phone geometry is not to increase drastically, the 5G smartphone will have much less space for the HDI PCB inside.

With the reduction in internal space for the PCB, and use of higher 5G frequencies, designers will need to exercise much stricter control on the impedance of traces. Unless they design with extreme precision, the thin traces in HDI PCBs can increase the risk of signal degradation resulting in lapses of data integrity.

PCB designers and fabricators are overcoming these challenges by following the mSAP process. Fabricators of IC substrates generally use this semi-additive process, and HDI PCB fabricators have adopted its modified version.

Typical line to space ratios on the HDI PCBs are 30:30, meaning designers plan for a spacing of 30 µm between adjacent traces of 30 µm width each. Demands of increasing density are forcing fabricators towards line-space ratios of 25:25 and even 20:20, with the help of mSAP. This enables makers of 5G smartphones and other demanding gadgets to achieve unprecedented densities while offering superior geometries with exacting impedance control for their high frequency operation.

Contrary to the subtractive processes used for normal PCB etching, mSAP does the reverse, essentially coating a thin copper trace onto the laminate and subsequently building up its thickness by electroplating over it.