How to Design Efficient High-Density Interconnect PCBs using Buried and Blind Vias

April 4, 2017 CircuitStudio

 

save space save money
The modern designer is always being pushed to go smaller and cheaper. Blind and buried vias can help you save space and money on your HDI PCB. Blind and buried vias help reduce outer layer usage, leaving more room for SMTs and BGA breakout channels. Read on to find out more!

Moore’s Law predicts that the number of transistors in integrated circuits (ICs) will double approximately every two years. This law has held true since 1965, but could be slowing down as transistors are shrinking at a slower pace. While Moore’s Law might be nearing its limit, the push for smaller and more powerful PCBs has only intensified. Fortunately for you, the modern designer has multiple techniques available to meet the space constraints of high density interconnect (HDI) boards. One way you can save space, and money, on your HDI board is by using a variety of different vias.

 

There are two main types of via that can save you space on your HDI PCB: blind vias and buried vias. To quickly refresh your memory, a blind via is like a through-hole via that ends somewhere inside the board instead of passing through completely. A buried via does not connect with either outside layer, and only connects layers inside the board.

 

computer motherboard
Prepare to shrink boards like these

 

How to Save Real Estate

 

The main reason to use blind and buried vias is to save real estate on your PCB. If you’re really good, blind and buried vias can even reduce an 8 layer board to a 6 layer board. These vias are especially helpful when it comes to saving space for surface mounted technologies (SMTs), particularly ball grid arrays (BGAs).

 

Blind vias can save you 50% more outer layer space than a through-hole via. Since only one side of the board is drilled, you can place a SMT component directly over the closed end of a blind via.  Now you have space for that last minute SMT component. In case you’ve been working too much overtime and haven’t had enough sleep, I’ll remind you not to place a SMT component over the open end of a blind via. Buried vias can be used in the same way to save space for SMTs.

 

Ever wished you had more space in the breakout channels for your BGA? If so, you probably should have wished for a million dollars or a new house instead. Anyway, blind vias and buried can make your boring wish come true. Not only do those pesky through-holes take up space for your SMTs, they also disrupt your BGA breakout channels. Just like with SMTs, you can use blind or buried vias instead of a through-hole via to widen your breakout channels. Depending on how much you can increase your breakout channel width, you may be able to lose a signal layer in your PCB.

 

save space, save money
Save space, save money

 

EMI Considerations

 

You should know by now that EMI is my jam. If you thought you were going to get through this without another lecture on it, you were wrong.

 

Blind and buried vias can actually help you reduce EMI on your PCB. You remember, of course, the other post where I told you about via stubs promoting EMI. In case you’re not a dedicated reader (you should be), I’ll remind you. The stubs on through-hole vias act as open-ended transmission lines and will reflect any signals transmitted through the via back into the circuit. The simple answer for this problem is to remove the stub. Blind and buried vias do not have stubs, so they won’t cause as much reflection.

 

Just because you can’t see a buried via, or the end of a blind via, does not mean they can’t cause EMI problems. Remember to follow electrical clearance and creepage rules when placing blind and buried vias.

 

blackhole
Next gen via technology

 

Manufacturing Considerations

 

It is extremely important that you talk to your manufacturer when designing your HDI with these fancy new vias. If you don’t consider manufacturing costs and limitations, you might end up with an expensive PCB that is filled with manufacturing defects.

 

To save yourself time and effort upfront, you should ask your manufacturer how they will fabricate these boards. Fabrication methods can limit your use of blind or buried. Your manufacturer will let you know what kind of vias their fabrication methods allow for.

 

In addition to the cost of various fabrications methods, you will need to think about the long-term viability of manufactured vias. Your manufacturer may suggest fabricating vias using the “peck drilling” method. Peck drilling is inexpensive but can introduce manufacturing defects into your PCB. Your manufacturer wants to save money, just like your manager. Make sure the corners they cut don’t cut the performance of your PCB.

 

If you’re designing an HDI PCB, your PCB design software should be able to deal with blind and buried vias. CircuitStudio has great documentation explaining how to handle buried and blind vias.

 

Don’t feel any pressure, but since Moore’s Law is slowing down, it’s up to you to increase circuit density and power. Use blind and buried vias wisely to save valuable space on your PCB. However, remember to design for electromagnetic compliance, cost, and long-term durability as well.

 

Having a panic attack about how to save space on your PCB? Talk to an expert at Altium.

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