How to Design High-Density PCBs With Buried and Blind Vias
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 Printed Circuit Boards 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.
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 Printed Board. 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 space on the outer layer than a through-hole via. Since only one side of the board is drilled, you can place an inner layer 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 on a printed circuit board. 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're in luck. Blind vias and buried vias can make your 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 Circuit Board.
Save space, save money
EMI Considerations on Printed Circuit Boards
Blind and buried vias can actually help you reduce EMI on your PCB components. 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 from the printed circuit board. 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.
Next gen via technology
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, you should ask your manufacturer how they will fabricate these boards upfront. Fabrication methods can limit your use of blind or buried vias. 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 printed circuit board 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 by undervaluing the connections between an outer and inner layer.
Since Moore’s Law is slowing down, it’s up to us to increase power density. Use blind and buried vias wisely to save valuable space on your printed circuit board. Remember to design for electromagnetic compliance, cost, and long-term durability as well.
Thinking about how to save space on your PCB? Talk to an expert at Altium Designer.