The Must-Knows for HDI PCB Fab

Why should I turn  to HDI?

At some level of circuit complexity, turning to an architecture with blind and buried vias will result in better yield and lower cost than would a through-hole design.

How does HDI keep my cost down?

One of the problems designers face is keeping cost down. Our goal is to provide some insight to help you with your decision-making process. When making PCB design decisions for HDI, one of the primary concerns is in regards to manufacturability and cost. Just like standard PCB designs, HDI boards have four main concerns:

  –  Materials

  –  Layers and laminations

  –  Vias in HDI classes

  –  Optimal trace and space

How do I select materials for HDI?

Materials play a large role in terms of manufacturability and direct cost of your circuit board. Here is a tip: The goal is always to select the right material for manufacturability that, at the same time, meets your temperature, and your electrical requirements. When it comes to materials, make sure that your high-speed material is also suitable for your HDI design. They are many other factors that come into play when selecting the proper materials for your design. Use our free Material Selector to find out which materials will suit your application the best.

What should I keep in mind when choosing layers and laminations?

When thinking about layers and laminations, keep in mind the following questions: Have you planned your layer sequencing for reduced EMI? Have you decided your routing density? What is the total number of sequential laminations? And have you minimized your laminations?

What should I keep in mind when choosing my via structure?

Manufacturability primarily has to do with via structure. Answer the following questions before you start working on your next project: Are you meeting the design guidelines for microvia aspect ratio? Are you stacking or staggering your microvias?

What should I keep in mind when choosing the trace and space?

And lastly, trace and space. Have you considered reducing your trace width to increase density and ultimately reduce your layer count? Here is a brief plug on Sierra Circuits: If you are considering reducing your trace width to increase density and reduce your layer count, 2-mil trace and space may be exactly what you are looking for. Our yields are 90% for this density.

When it comes to the manufacturability and the cost of an HDI board, it is very important to remember that none of these considerations can be dealt with in isolation. For example, reducing your trace and space will always ultimately reduce your layer count. Or, how can you plan your via and pad sizes which impact the space you have to route your traces?

What is the most expensive stack-up class?

The more sequential laminations, the higher the cost. The most expensive stack-up class is three sequential laminations. This includes microvias stacked on top of each other, which is necessary when you are breaking out of a tight pitch BGA, like .3 mm. Second in the stack-up class in terms of cost are using a non-conductive hole through process.

How do I choose between mechanical or laser drill?

Drilling is the huge cost driver. A lot of factors can impact your decision. To name a few, there is the cost of the drill bit, the time it takes to drill mechanical holes, and the quality of the drill. A 6-mil drill is much more expensive when compared to a 10-mil drill. What can make this process even more complex is the use of hybrid materials. For hybrid materials, each material requires a different drill cycle and plasma cycle. Bottom line, if you have the opportunity to reduce or eliminate mechanically-drilled vias and use laser-drilled microvias instead, you could save a tremendous direct cost for yield.

How do I choose between conductive and non-conductive hole fill?

Do you have vias underneath a surface-mount component that is required to be filled and capped or plated over? It is costly because it requires two plating steps and two drill steps. We drill holes that will be filled separately from the regular through-holes on the board and any extra steps mean extra cost. In this case, we are going back to the process twice, which costs in terms of time and dollars, drilling and plating.

What are the critical dimensions of a laser-drilled microvia?

The critical dimensions are the pad size, the laser drill size, and the press-out thickness of the dielectric. You must define your laser drill size in proportion to the finished press-out thickness in order to properly plate the via. Laser drill dimensions can get very small. But remember, they are only going from one layer to the next. Many fabricators can laser a 2-mil hole but plating the 2-mil hole becomes problematic because of the aspect ratio of the thickness due to the drill diameter of the hole.

What is the optimal aspect ratio?

You want to stay 0.75:1 to ensure a good plating. The shape of the microvia is important to allow for the plating solution to flow properly and plate or fill the microvia. If you have to do build-up, or in another word you need a buried via, then you would need to choose between staggered or stacked vias.

What is the difference between staggered and stacked microvias?

Staggered vias essentially mean fewer process steps. We do not have to fill the laser-drilled vias with copper because the second laser drill does not land on the first laser drill. Filling or plating a microvia shut usually happens in a special plating tank designed with chemistry that plates the laser-drilled microvia from the bottom of the via to the top of the via, until it fills the hole completely. Plating a laser-drilled microvia shut adds time and cost to the process. You only need it when you are stacking on an inner layer. Or if you have a via-in-pad on the outer layer. If the second laser-drilled via is staggered or offset, there is no need to copper plate shut. If you are staggering your laser-drilled microvias, it is important to know what spacing your manufacturer requires between the laser drills.

What is the laser drill accuracy?

It is very safe to assume it is +/- 1 mil accuracy. Usually, in a staggered microvia formation, the diameters of both operate and lower microvias are the same. The key parameter that decides whether the staggering is possible or not, without the lower microvia needing to be filled, is the dimension E, the vertical separation between the central access of the two microvias. For staggering to be viable, the value of E must be greater than the microvia diameter.