How to Determine the Number of Layers in PCBs

Introduction:

 

Printed Circuit Board (PCBs) are the essential part of any electronic product that we see today in our daily life. The hard core green color boards called PCBs are the primary base upon which the functionality of electronic product is based. However these PCBs are of numerous variety and are available in different sizes, shapes and colors. The PCBs are commonly seen in electronic toys, home appliances, office automation electronics, industrial automation, computer and IT industry, telecom, military and aerospace and defense industries.

 

These PCBs are the core of electronic instruments found in electronic laboratories, medical devices like ECG, MRI, CT scan, Radiology equipment etc. Also PCBs are the building blocks of latest state of the art equipment of military, ground segment, Radar electronics, Sonar, navigation, Aircrafts, Fighter jets, Spacecraft.

A Typical 4 Layer PCB Stack Up

 

A commonly asked Question:

 

It has been seen many times that customers ask question “Whether I should go for 2 sided PCB or multilayer PCB?”  In my opinion the answer to this question is not easy but also not very difficult. This is because the customer should be very clear about his or her requirements of the end product as where this PCB is going to apply. This question generates the basis upon which the designer will decide whether to go for 2 sided PCB or multilayer PCB. The key thing is the application of PCB or where the PCB will be used.

 

For example if a customer wants to build an Audio amplifier circuit then he puts his early requirements in front of the design engineer. The design engineer will then check how many functionalities are required to make a complete audio amplifier. The functionalities will be 1- Pre-amplifier circuit 2- Band pass Filter circuit 3- Tone Control adjustment 4- Audio level or gain control 5- Power amplifier with 8 ohm or 16 ohm speaker driver circuit. Now as there are many functions in the product hence more components are required and hence more PCB space will be required. Now if the customer wants the product to be small in size then the multilayer PCB is inevitable.

 

Now we can say that the main reasons for multilayer PCB selection is the size and complexity of circuitry. The following are the main reasons that determine whether to use 2 sided of multilayer PCB for your product.

 

  • Application of Your PCB:

The critical point that determines the number of layers of your multilayer PCB is the usage or application of your PCB. The more the high tech application like medical electronics or military specifications than more layers will add up.

 

  • PCB Size:

Now if you do not care how large your board becomes instead your circuitry is complex then it is possible to place all components on one single large double sided PCB but it will make the PCB very large and hence not pragmatic to be used in any application. On the other hand if your application demands compactness like ECG machine processor board, glucose level measurement devices, microprocessor and memory interface circuits or even the lightest and handy smart phone very popular nowadays then obviously the PCB small size is desirable. Hence the circuit designer has to select SMT components, BGA and QFN packages for ICs and placement must be optimized and placement of vias and interconnects must be carefully dealt so as to use minimal board area. This will result in a multilayer architecture of PCB with High Density Interconnected components (HDI) PCBs.

 

  • Operating Frequency:

The most high frequency or high speed digital circuits are based on multilayer PCB, so as to avoid signal interference from surrounding noise sources like 50/60 Hz power line, power supplies, audio or other low frequency distortion source. The multilayer PCB with separate signal layers ensures high speed signal integrity by reducing signal line inductance and parasitic capacitances.

 

  • Your Budget:

Above all things comes your budgetary constraints. Most of the times individual hobbyists and student do not have much money to invest in expensive multilayer PCB fabrication prototypes hence they opt for double sided PCBs. But again if the design is too complex or it contains high speed components and EMC and EMI considerations are necessary then multilayer is desirable. For larger companies having reasonable budgets and who are looking to grow their business usually consider quality PCB and overlook price. So being individual or hobbyist or student if your project demands multilayer PCB, then you should go for it and must pay for the high manufacturing cost of multilayer PCB setup.

 

  • Lead Time:

Another important factor that most of us ignore is the lead time. For example a 4 layer PCB with lead time 15 days will be more costly than 4 layer PCB with lead time 30 days. In other words we can say that if the time required to manufacture and ship to your location (lead time) a 4 layer PCB is 15 days then the time required for 6 layer PCB will be 18 days and for 8 layers it will be 25 days and so on.. This is not exact but just to give you an idea that as layers grows hence the lead time also grows.

 

Single Layer Vs Double Sided:

 

People also get confused while considering single sided or double sided PCBs. Just to clarify that a single sided PCB or single layer PCB is the one that has components on only one side (either top or bottom) and other side has solder joints. While the double sided PCB has components on both sides Top and bottom. This selection between the two again is governed by the fact that where your PCB is used. If your PCB enclosure has space constraints then you should go for single sided. Like in old FM radio circuits, calculators and small electric circuits used in kitchen appliances like electric kettle or boilers. Mostly THT (Through Hole) components are mounted on one side and other side is solder joints. However again if your components are more and you need to adjust components somehow then you can place components on top and bottom layers both and then use PTH (Plated Through holes) to interconnect components on top and bottom layers.

 

Conclusion:

 

It can be said that while choosing number of layers and choosing between single, double or multilayer PCB, it is important that you know space constraints of your PCB enclosure, dimensions of PCB matters a lot and how much complex your circuit is also governs the PCB number of layers.