I would never be accused of being, or even remotely resembling, a chef. However, I do manage to produce an edible meal from time to time. One of the things that I found hardest to grasp was the fact that not all ingredients are included for taste. In fact, some ingredients are included for preservation and others merely for appearance. The former is why a lot of foods include some form of salt, and the diversity of flavors for some items is owed to food coloring. Perhaps, being a good chef requires you to understand why you are adding a particular ingredient to a dish.
Being a good PCB designer means that you know how to design the stackup. And designing a good stackup requires that you understand what the elements or layers should be and how they should be arranged. There are many factors that should influence your choices and design, including board type, layer numbers and arrangement, and material properties. Although the electrical and thermal material properties are important to your board’s design, you should ensure that the mechanical properties are incorporated in the design of the stackup. Before discussing how to do this, let’s take a look at the PCB mechanical properties that should be included.
PCB Mechanical Properties You Should Know
The PCB stackup, comprehensively, refers to how the signal and plane layers of your board’s construction are mechanically arranged and connected. During PCB construction, the layers are “stacked,” typically in symmetrical order from top to bottom. This arrangement is usually guided by the types of signals that will propagate through the board and the via selections that will facilitate the signal and current flows. Mechanically, your board needs to be able to withstand any physical stresses that it may encounter during the manufacturing process as well as normal operation. During assembly, these include temperature variations encountered during the soldering and reflow processes and bending stress during depanelization and other handling. Operational hazards may be due to vibration, inclement weather conditions, or in the case of critical systems, such as aerospace, extreme temperature swings and radiation.
In order to counter these potential threats to your board’s structural integrity, you should be aware of the mechanical properties that provide you with a quantifiable assessment of how your board will fare under mechanical stresses. The most important of these are:
- Bending strength
Probably, the most important mechanical property of your board is how well it resists forces that may alter its shape regardless of form factor. A board’s bending or flexural (a term usually applied when discussing flex or rigid-flex PCBs) strength is most challenged by exposure to excessive heat or direct physical force; however, shock and vibration may also be threats with high force or pressure. Another strength measure that is sometimes used instead of bending strength by some manufacturers is Young’s modulus, which quantifies a material or structure’s stiffness, especially in the longitudinal direction.
- Time to delamination
This property defines how long your layers can withstand heat beyond a certain threshold value before the laminate will separate from the substrate. This is important for assembly, as reflow temperatures may reach 250°C (482°F) for lead-free soldering or if your board will be exposed to extreme temperatures for prolonged intervals during operation.
- Peel strength
This is related to the time to delamination. Here, however, the minimum bonding pressure is the definitive metric. Threats to this bondability may be thermal or chemical. The peel test is one of the circuit board testing methods that can be performed during manufacturing.
Density is another important property of your PCB that is typically associated with the dielectric material. The denser a material is, the less susceptible it is to breakdown due to mechanical force. Dielectric density and thickness are also important in setting your board impedances.
Understanding the above mechanical properties and how they impact your board can be used to help you design your PCB stackup.
Stackup Design Using PCB Mechanical Properties
Typically, the stackup is designed from an electrical perspective where the number of layers and their configuration are the primary objectives. Certainly, these are very important and the selections made will determine how well the PCB handles signal propagation and meets the operational criterion for the design. However, operational success begins with the manufacturability of your board and can only be sustained if your PCB is able to reliably perform within its deployment environment. These objectives are best met when you incorporate PCB mechanical properties into your design. This can be done by following the suggestions listed below.
1. Know the operational environment in which your boards will reside.
In order to make good selections for your board materials, it is necessary that you know the environmental conditions to which they will be exposed. This includes whether your boards will be subject to shock or vibration, in addition to the range of temperatures. For example, boards that will be included in moving machinery will need to withstand continuous shock and vibration that must be considered when selecting materials to guard against failure in the field. Equally, important is choosing materials that will not be compromised due to temperature variations, such as may be present in automotive, aeronautical and industrial systems.
2. Make sure that your boards meet or exceed bending strength requirements.
It is a given that your boards must adhere to the regulatory requirements of IPC-6011, IPC-6012, and IPC-6013, based upon the type and classification. These standards include strength requirements for rigid, flex, and rigid-flex boards. In the case that your PCB must adhere to higher standards, this must be considered when selecting materials. For example, if your boards will be a part of an automotive or aerospace system the strength requirements will be higher than for commercial applications.
3. Utilize peel testing to ensure your boards will not delaminate.
Delamination is a concern during assembly, as well as once your boards are in the field. To ensure that your laminate bonding is satisfactory for these conditions, you should make use of peel tests during low-volume manufacturing. Doing so will allow you to make material changes prior to high-volume runs that may have low yield rates. Peel tests are a good way to evaluate your board’s response in various environments as it provides a specific metric that can be compared against the environment’s requirements prior to deployment.
Following the suggestions above will help you minimize the possibility of running into mechanical problems during manufacturing or after installation. In conjunction with these, you should institute a structural integrity testing regimen.
Good PCB stackup design involves more than ensuring your signals are properly routed and grounded. It also requires that you ensure your materials and parameter choices include consideration of how the PCB mechanical properties will impact your board’s assembly and ability to maintain structure in its operating environment.
|Tempo‘s Custom PCB Manufacturing Service
At Tempo Automation, the PCB industry’s fastest turnaround high-quality manufacturer for prototyping and low-volume production, we will assist you in making the best selections for your design.
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 user, you can simply add these files to your PCB design software.
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 mechanical properties or how to incorporate them into your PCB stackup design, contact us.