Have you ever tried to complete a puzzle without looking at the picture on the box? Not only does it take an extremely long time, but it’s also pointlessly frustrating. Prototyping a PCB for the first time can feel the same way. The design can be perfect but there still may be problems and hidden pitfalls that will complicate the entire process. The first thing you’ll need to be aware of is panelization, which can be required if the board is oddly shaped. Fiducials are also normally needed so that the CM’s equipment can accurately locate a board on the line. While both panelization and fiducials are critical, other decisions; such as controlled impedance and PCB stackup that affect your material selection and trace routing are also important considerations. However, the most important step in the process is communicating with your CM early and often. After we’ve delved into these topics, you’ll be sure to have a smoother prototyping experience.
What is Panelization?
Imagine this. You’ve reviewed design rules, checked the lifetime and supply of components, and even examined which materials to use on your board. You send the design off to your CM, only for them to tell you that the board as designed will result in unused panel space and wasted materials! At this point, your options are to re-design the board and/or change its shape such that it makes better use of the standard board sizes for PCB manufacturing (aka “panelization”).
Panelizing a PCB after you have already settled on a design can be costly and increase your turnaround time. Communicate with your CM from the beginning and see if your board needs to be panelized. If so, there are several things to consider during design.
- Depanelization - Obviously, you can’t sell your boards in one giant array, so they have to be broken out of the panel after assembly. Typically CMs can use either routing or scoring for depanelization. Routing uses a bit to cut away material and form tabs that connect the board to the array. Scoring cuts a v-shaped groove into the array that allows PCBs to be broken out at the end. Routing is more complex and thus more expensive than scoring. To actually break the PCB out of its panel, you can do everything from manually removing it to using a laser to cut it out. Talk to your CM about which methods can be used with your panelized array.
- Edge Components - Try to avoid placing sensitive SMT components near the edge of the board or have connectors that may hang over the edge. If you absolutely need to have connectors on the edge, then your CM may need to use routing for depanelization.
- Shape - Panelizing PCBs is very much like putting together a puzzle. If all the boards are rectangles, it can be relatively simple. If you have more complex shapes, then you may have to get creative. You can try rotating your boards for more optimal placement.
Panelization is a requirement for many boards these days. If you don’t take it into consideration from the beginning you’ll be looking at expensive re-designs and delays in your schedule.
Fiducials in PCB Prototyping
Not to be confused with any kind of fiduciary, fiducials are marks made on objects so that machine vision systems can recognize them. In PCB prototyping, these are used to find the alignment of boards for picking and placing during assembly.
Fiducials can have many shapes but the most common are diamonds, squares, or circles. Don’t try and get creative here; an oddly shaped fiducial will increase the cost to manufacture your board and adds nothing to your design. Fiducials are usually placed on the top or bottom copper layers of a board and should not be covered up by solder mask. This will make them easy for machine vision systems to see. They should also not be placed near any other feature that looks like the fiducial, or the cameras may mistake one for the other or visa versa.
It’s usually a good idea to place at least three fiducials on your PCB. This will help the vision system to determine the exact orientation of your board. Avoid placing fiducials too close together; the farther apart they are the better they will be recognized. This is something that may seem superficial since the board doesn’t actually use the fiducials, but they are critical for assembly. Contact your CM to see if they have any specific requirements for fiducials that will work with their systems.
Important Prototyping Considerations to Discuss With Your CM
When prototyping a PCB, the most important thing to do is to communicate with your CM early and often. This not only helps with PCB panelization or fiducial design and placement, but will also make the entire process much easier and less expensive. When your CM is involved from the beginning they can help you with decisions; such as controlled impedance, board stackup, and even which solder to use.
Many circuits require controlled impedance to maintain signal integrity. If you like math, then you might want to calculate all the trace thicknesses, dielectric constants, etc. to get the impedance you want. The other option is to specify what impedance you want and work with your CM to find layouts and materials that work.
When it comes to PCB design, the stackup matters. Signal layers, power planes, grounding planes, analog systems, and digital circuits must be carefully arranged to maintain signal integrity, ensure mechanical strength, and control impedance. Your CM should be involved in board stackup as well because they may or may not be able to use certain materials in the manufacturing process. Before you specify a material because of its dielectric constant or physical resilience, you need to confirm it can actually be made.
Another point to discuss with your CM is solder. If your board is destined for the European Union, then you’ll likely need to use lead-free solder. Your CM will need to know what kind of solder you have to use since it will affect what processes they can use. The number of through-hole and SMT components will also affect manufacturing so be sure to talk that through with your CM as well.
All in all, PCB prototyping can be a pain-free process if you know what to expect. From the beginning, you’ll need to think about panelization and depanelization. It’s also worth considering fiducial design and placement in the design stage, rather than when you’re about to get a prototype made. The most critical thing to remember is to communicate with your CM about your design. Things like desired impedance, board stackup, and solder will affect parts of their process and will impact your turnaround time and cost.
|Tempo‘s Custom PCB Manufacturing Service
Although regular communication with your CM is important, it will only help if you have a good one. That’s why you should work with an industry leader like Tempo Automation.
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.