If you asked most people what the first greatest invention our ancestors discovered (or stumbled upon) was, “fire” would probably be the answer. You certainly cannot dispute its importance as a means for cooking food, illuminating the dark, purifying water, and providing warmth against the cold. The first and last of these usages are still prevalent today. However, as good as fire or heat is in protecting us from the cold, it can also be dangerous. Thus, the need for for the invention of insulation, which protects us from both the cold and the heat. In recognition of the importance of insulation, it is now mandated for virtually all structures where people reside.
Insulation is also important for circuit boards. Just as for housing, it provides protection against potentially dangerous temperature changes. For PCBs, the insulation is the dielectric material used to separate and isolate conducting elements and layers. It is critical that you understand your board’s construction and the mechanical properties of the materials used to ensure the highest quality manufacture (and thus the best operation). This is best understood if we first take a closer look at PCB insulation between layers and then discuss how it impacts board assembly and operation.
What is PCB Insulation Between Layers?
PCBs are generally referred to according to the number of signal layers that they contain. For example, a four-layer PCB will have four layers that carry signals, as shown below.
Four-layer PCB structure
In the figure above, the signal layers are the top, bottom, layer 2, and layer 3. These layers may carry analog, digital, and/or DC signals. Additionally, layers 2 and/or 3 may be ground planes. The core and two prepeg are also layers, but not signal layers. These provide insulation to isolate adjacent signal layers. The actual material composition of this insulation depends upon your selections, which should be based on your board type.
Although choosing the number of signal and insulation layers, their arrangement, and types of materials are tasks for the PCB stackup design, the CM often makes these decisions. This is due to the fact that making the best choices for insulation depends upon your knowledge of the electrical, mechanical, thermal, and chemical properties of your board's materials. To achieve this, we need to define some objectives for PCB insulation between layers, as listed below.
Objectives for PCB insulation between layers
- Minimize or eliminate signal interference between adjacent layers.
- Maintain adhesion during the assembly temperature cycling.
- Aid in thermal distribution during assembly.
- Aid in signal and power integrity.
Now, let’s see how we can use the PCB insulation between layers to meet these objectives.
DFM for HDI Printed Circuit Boards
How Insulation Helps PCB Assembly and Operation
Although there are still some cases where single-layer boards are used, today, most PCBs are multilayer due to their compact size, higher quality, and propensity to accommodate more complex designs. These advantages do present other challenges, including more complicated signal routing. Instead of clearance and creepage on the board surface being the primary signal integrity issues for operation, the use of multiple layers and vias introduce PCB insulation between layers as a consideration. For manufacturing, multiple layers mean multiple adhesion points and potential lamination separation. In particular for assembly, increased board height and vias present heat and soldering distribution issues. However, these can be successfully addressed by making selections that meet our PCB insulation between layers objectives. Considerations for material selections should include the following:
- Insulation thickness and layer signals
If thin insulation is used then having the ground layer adjacent to a signal layer adds isolation between the signals and reduces EMI issues. However, when a thicker dielectric should be used, two signal layers may be routed on adjacent layers, provided traces on one layer are routed perpendicularly to routes on the other layer.
- Type of board
For most board types, common FR4 will probably suffice. However, in many cases, especially for high-speed boards, controlled impedance may be a major factor. In these cases, the height of insulation is important and must be chosen to meet the impedance requirements.
- Overall board height
As multilayer boards are utilized due to their size, your choice of insulation height may be bounded by overall PCB thickness. Accommodating this restraint requires coordination between the number of layers, insulation type, and height.
- Thermal Cycling
Another important consideration of how many times your board is exposed to high temperature cycling during manufacturing. For example, flex type boards typically require many more cycles than rigid boards.
When designing circuit board stackups, the PCB insulation between layers is often not given the attention that is warranted. However, by incorporating a few important considerations, you can employ insulation that not only supports operation but aids the building of boards. At Tempo Automation, we can help to ensure that you make the best use of design tips such as these, regardless of the type or complexity of your design. This is in addition to ensuring that you receive the highest quality boards with the best turnaround in the industry.
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And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on PCB insulation between layers or how best to use it in your design, contact us.