Join our next Ask Me Anything about blind and buried vias where you can ask us questions on:
• Design rules and constraints
• Benefits of HDI
• Cost implications
• And much more!
Question submission window: Through September 4th
Can you explain what impact blind and buried vias have on the overall cost?
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On our last high density board we had misalignment issues between blind vias and the corresponding pads on an internal layer. What would you recommend to improve alignment, and how do you handle tolerance stacking?
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The use of blind vias in one of my RF designs resulted in unexpected noise. Do you have best practices for using blind vias in RF circuits?
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Is there a quantifiable signal integrity difference between using staggard vs. stacked blind vias?
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Which is cheaper to produce and by what percentage: adding vias that go halfway through the board or adding more microvias?
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Thanks everyone for posting your questions! Our experts will reply tomorrow.
I can’t think of how the via itself would create noise, so if the noise is real then it must be getting coupled in from something nearby. Don’t know of any but the standard guidelines for blind vias and RF. Avoiding any vias is great if you can.
Just due to geometry I would guess the staggered must reflect a less uniform impedance but I suspect the differences are so small it seems unlikely you would see them.
@Timmann17 and @Kevin_SD your questions are pretty close so here it goes:
This is a basic overview for both builds but the example on the left is somewhat easier to produce.
• It would start with two sub-assemblies: 2 - 8 and 15 - 9 each with plated through holes.
• These would be laminated together to form a 2-15 sub-assembly with 2- 15 through holes and 2-3 & 15-14 laser vias. All the vias get plated along with the image on layers 2 & 15.
• Final lamination for layers 1 & 16 with 1- 16 through holes and 1-2 & 15-16 laser vias.
• Bottom line is 4 lamination cycles, 4 through hole drill programs and 4 laser drill programs.
The example on the right:
• Starts with a layer 5- 12 sub-assembly with a 5-12 through hole and 5-6 & 12-11 laser vias. All the vias get plated along with the image on layers 5 & 12.
• Next starts the sequential lamination of layers 4 & 13, 3 & 14, 2 & 15 and 1 & 16 with laser vias on each layer. Each set of layers requires it’s own press cycle.
• As the board gets built out from the center core the vias and image for each layer get plated.
• There is a final 1 – 16 plated through hole.
• Bottom line is 5 lamination cycles, 10 laser via programs and 2 through hole programs.
Note on Laser Vias – both examples show staggered vias which don’t require fill plate and can be plated at the same time as the layer image and any through holes. If they were stacked it would require fill plating and planarization which adds quite a few process steps and cost.
Can’t really speak to cost but each lamination cycle basically represents a finished board so probably not a huge difference between the two. The big difference is how much space the 2-8 & 9-15 through holes take up as opposed to the laser vias where you can run circuitry under them.
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This gets a bit complicated but first of all the cores shrink to varying degrees during lamination based on material type, thickness, copper weight, etc. so it’s easy to have core to core registration shift and the internal pads don’t align. To overcome this we use a software system in conjunction with an X-Ray drill to accurately predict and measure material movement which greatly improves layer to layer registration. The X-ray drill is also used to drill alignment targets tied to internal fiducials that follow any material/core movement. These targets are used by the machinal vision drill and laser drill systems to accurately align to the internal registration. If multiple press cycles are required the panel will shrink during each cycle and without these systems in place it’s almost impossible to maintain hole to pad registration especially when trying to land a 5 mil laser via on a 9 mil pad. It’s also quite difficult to align all the mechanical drill and laser drill programs usually found in HDI builds without these systems.
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