Modern electronic modules demand increasingly high levels of density. The miniature sizes of most surface mount devices (SMDs) that have now reached unbelievably small dimensions are primarily fueling this demand. Other advancements such as improvements in packaging technology has led to fine-pitch components such as the Ball Grid Array (BGA) sporting more than 1500 pins and a 0.8 mm pitch on average, covering entire bottom surface of the package.
Microvias, Buried Vias and Sequential Build-up
All the above has led to an unprecedented reduction in the widths of structures on the PCB, that is, the track widths and the spacing between them. To improve the density further, fabricators are using microvias, buried vias, and a sequential buildup of multi-layer boards to improve the integration. Use of these technologies offers designers more space on the outer layers for placing components. For instance, use of buried vias precludes the use of through-holes, which would have to go through all the layers.
Use of microvias, buried vias and fine structures results in boards with very high-density interconnection (HDI), and Sequential Build Up (SBU) of multiple layers supplements this. For instance, an HDI-SBU multi-layer board will have at least two layers or a multi-layer core and one or more external layers with microvias.
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Advantages of HDI-SBU Technology
PCBs with standard lamination and through vias are less expensive to manufacture, as they have simple via models and use ordinary dielectrics such as FR-4. The manufacturing process is mature, and fabricators can produce high-reliability boards.
As the layer count increases, fabricators start facing problems with boards using standard lamination and through vias. The costs skyrocket, and few fabricators can reach good yields. They face difficulties such as delamination at high temperatures, increase in layer count as large vias reduce the ability to route, implementing BGAs with pin pitch below 1 mm, capacitive coupling from through-hole vias, long via stubs creating impedance mismatches, and many more.
HDI-SBU boards solve most of the above problems. Microvia technology makes vias much simpler to fabricate, with potentially shorter via stubs. As microvias are dimensionally much smaller than through-hole vias are, designers have much more area for routing traces, thereby offering them the only practical way of implementing multiple fine pitch components such as the BGA.
Advantages in relation to the Manufacturing Process
With smaller feature sizes for traces and vias, HDI-SBU boards enable designers achieve much higher route densities than before, resulting in fewer number of layers. Therefore, although the HDI-SBU manufacturing processes are more expensive compared to the cost of fabricating standard boards, the high-density boards can be made in much smaller sizes and lower number of layers, thus helping to offset the cost. In fact, multi-layer HDI-SBU boards with larger number of layers are cheaper to manufacture compared to standard boards with the same number of layers. Moreover, newer materials are now available, making HDI-SBU boards compatible with RoHS processes, offering better performance at high temperatures, and improved signal and power integrity at high frequencies.
Applicable Standards for HDI-SBU
Japan Printed Circuit Association publishes IPC/JPCA-2315, jointly with IPC, and these standards provide easy-to-follow tutorials on selecting HDI and microvia design rules and structures.
The team at PCB Global are able to assist with any queries you may have in relation to if HDI-SBU PCB’s are necessary or beneficial to your PCB specifications, requirements and outcomes. Please feel free to contact the team at firstname.lastname@example.org any more information on this area of PCB manufacture.