Some customers would be curious when ordering: why the layers option are all even numbers, why can't they be odd numbers?

For this question, I guess many guys have searched it from Google, just like me. To be honest, some answerers from Google aren’t rather professional in PCB this field. 

Theoretically, as long as it is within the process capability, no matter what PCB design layer is even number or odd, manufacturers could process them all.

But, online or offline, unless you paid a great deal amount of money to let manufacturers handle orders of odd-numbered layer PCBs. Otherwise, they won't consider those orders.

Why?

It’s pretty complicated to explain this. The core issues are mainly: Cost, time, and quality.

Cost

Firstly, on the basis of nuclear structure technology, the odd-numbered PCBs also need the non-standard laminated core layer bonding process. Therefore, the cost of it is much higher than even-numbered layer PCBs.

Moreover, for many brokers, the design of even-numbered layers belongs to universal design while odd-numbered layers belong to special designs. The special design means it has to use the special production process to match particular, even completely different production parameters, which takes lots of cost of trial and error.

One more, brokers won’t take the special design as a common production process. Because the process going on must be with professional and designate engineers, which results in naturally rising labor costs. What’s more, only the special design be discussed with production, technology, quality, and other departments and everybody work together did it could be implemented successfully.

All of the above factors lead to the expensive cost of special design PCBs. Hence, the odd-numbered layers in PCB are more expensive than even-numbered layers of that. That’s why most customers can’t accept the price and change their options to standard even-numbered layers.

Time

Except for cost, the odd-numbered layers' PCB production time would take much more than the even-numbered ones.

On one hand, special design means it is different from normal design in many ways. For odd-numbered layers of PCBs, brokers need to make a serial of pre-arrangement so that make it manufacture smoothly. This gonna need more workload and time.

Additionally, special design is so unfamiliar for workmen that bring about excessive production time.

So for some manufacturers, there might need only 2 days to deliver 4-layer PCB, while 5-layer ones based on special design take 5 days or even more.

Thus, though brokers would rather produce batch orders of special design products, customers would usually give up orders for long-term production.

Quality

Money and patience could solve problems from all of the above, but quality is still the most concern that everybody cares about.

The best explanation for cancel producing odd-numbered layers PCB is to ensure the quality:

1. Odd-numbered layers PCB is easier to bend. When the PCB passes the multilayer bonding process and finally cools down, the PCB would go bent for differences in lamination tension of core and foil structures. The key to eliminating the bend factor lies in the use of the balanced laminate. The even-numbered layers PCB is symmetrical, and its warpage degree could be controlled under 0.7%; whereas the odd-numbered layers PCB, especially the big size one, its warpage degree gonna over 0.7%. 

2. The laminate design of even-numbered layers is beneficial to improving the PCB’s anti-interference ability. Signal layers are always paired with ground layers or power layers, which could shield the component's signal interference. The common layer order of 6-layers PCB, such as SIG-GND-SIG-PWR-GND-SIG.

And there is copper on both sides of one board, which means both sides could be filled with circuits. However, creating an odd-numbered layer PCB is meant to waste, cause there has one blank layer without any circuits.  Hence, for every EMI engineer, they would design this blank layer as a power layer or ground layer for playing a shielding role.

One more, it is hard to ensure the quality of special design products. That’s why the whole quality of odd-numbered layer PCBs is lower than that of even-numbered layer ones. And it is also to do all-round monitoring during the production. No matter which broker it is, the development of qualification rate and the quality needs a long-term.

To conclude, brokers don’t like these difficult and high-cost orders, and customers aren’t willing to invest too much in these orders. So a hidden rule is gradually formed:

Everyone defaults to using the lamination method to produce high-level multiple-layer boards and the boards are designed to be even-numbered layers, which not only reduces the price but also facilitates the processing of good products for the first time.