Why is “Panelization” performed during PCB manufacturing? And after surface mounting (SMT) assembly and wave soldering, why is it necessary to go through the additional process of “de-paneling” to separate the individual boards? What is the purpose of the PCB’s break-away edges or coupons? Isn’t it cost-effective to use less material? What exactly is meant by “material utilization rate”?
Why is “Panelization” performed in PCB manufacturing?
In general PCB production, a process called “Panelization” is carried out with the aim of enhancing the efficiency of the Surface Mount Technology (SMT) or wave soldering assembly lines.
PCBs often come in configurations known as “multi-up” boards, such as 2 in 1, 4 in 1, 6 in 1, and so on, where multiple individual boards are combined into a larger single panel.
If you ever visit an SMT assembly line, you’ll notice that the main bottleneck lies in the “Solder Paste Printing” process. Regardless of the PCB’s size, the printing process typically takes around 25 seconds. This means that if the subsequent high-speed pick-and-place machines or general pick-and-place machines finish their operations faster than the solder paste printing machine, there will be periods of idle time. From an economic standpoint, this idle time is considered wasteful.
Modern pick-and-place machines are incredibly fast, capable of placing multiple components within a second. Some machines even have multiple nozzles working simultaneously. Considering the number of components on today’s mobile phone PCBAs, if only a single board is processed, all the placement work could be done in less than 10 seconds. Therefore, using panelization to increase the component count during pick-and-place can optimize the utilization of these machines and improve overall efficiency. Ideally, achieving a “Line Balance” would ensure that every piece of equipment operates at its maximum capacity.
Additionally, in certain cases with unique design requirements, most components might be concentrated on one side of the PCB, leaving only a few components on the other side. In such situations, the use of mirrored panelization, also known as a “mirror board” where positive and negative board at same side, can be considered to increase the component count on a single side of the PCB. However, mirrored panelization comes with certain limitations.
Panelization of PCBs also offers another advantage in terms of time efficiency during handling of PCBA (Printed Circuit Board Assembly). Multiple boards can be retrieved or placed at once, saving time. If panel-level testing can be implemented, it can significantly reduce the labor hours wasted in loading and unloading boards from fixtures during subsequent board-level testing.
Lastly, some PCB designs can take irregular shapes. For example, for a board shaped like an “L,” if it is produced using “Solution 1,” there will be excess waste material in the top-left corner. If this can be changed to a mirrored panelization where the orientation is flipped top to bottom, waste material can be greatly minimized. This is the concept of enhancing PCB material utilization.
Disadvantages and Limitations of PCB Panelization
While PCB panelization offers numerous advantages, it still requires the separated boards to be cut (de-paneled) into individual units once all PCBA assembly operations are completed. This introduces an additional step in the process, leading to increased labor hours and an elevated risk of component collisions during transportation.
For boards containing fine-pitch components like 0201-sized parts, there is a limit to the number of boards that can be panelized together. The panel width or length cannot be too extensive due to PCB material expansion and contraction. Consequently, variations in dimensions will inevitably occur between individual boards within the panel. If panelization involves too many boards, the resulting tolerances may be so significant that they cannot meet the precise requirements of solder paste printing. This could result in solder paste misalignment, leading to soldering issues.
Panelizing a large number of thin PCBs is not recommended either, as thinner PCBs exhibit more deformation. When a higher number of boards are panelized, the panel’s width increases. This presents challenges for surface mount component placement and reflow soldering. While this issue can be addressed using carriers or full-process carriers, it’s necessary to consider the cost of these carriers and the added labor expenses.
What is the Purpose of Creating Break-away Edges for PCBs?
The purpose of creating break-away edges, also known as PCB coupons, is to aid in PCB assembly production. While some refer to these edges as coupons due to their resemblance to tear-off discount coupons, I strongly recommend against manually breaking off excess break-away edges for quality reasons. Instead, it’s advisable to use machine equipment (such as Scoring or Router) to remove these edges. This helps to minimize stress during the breaking process, preventing potential impacts on the PCB and components like solder cracking or component fracture.
The primary function of designing break-away edges on PCBs is to facilitate the PCBA assembly process. Modern SMT assembly lines are highly automated, relying on conveyors and chains for PCB transportation. As you might have guessed, the main purpose of break-away edges is to provide a pathway for these conveyors and chains to transport the boards.
Of course, if you choose to, you can also leave a certain area around the PCB empty of electronic components. It’s generally recommended to maintain a margin of at least 5.0mm. This is because the chain of the reflow oven requires some space, and by leaving this space, you might have the opportunity to avoid designing break-away edges. Otherwise, the belts and chains might damage the electronic components around them.
Additionally, the edges of PCBs serve other purposes:
They can accommodate “Tooling Holes” for positioning the board during In-Circuit Testing (ICT) or Functional Verification Testing (FVT) after the SMT process. This helps prevent misalignment of the test probe and test points.
They can be utilized for detecting the position of the circuit board on the SMT assembly line. Typically, each machine on the SMT assembly line is equipped with a sensor to detect the arrival of subsequent boards. This ensures that only one panel is being processed inside the machine at a time. Without this safeguard, if a new panel enters before the previous one is completed, confusion could arise, leading to incorrect processing. Some PCBs might have their shapes in a way that leaves an opening where the sensor detects, usually at the top-left corner of the board’s leading edge. In such cases, the board edges can be utilized to provide a solid area for the sensor’s accurate detection.
In situations where there are limitations in placing numerous fiducial marks inside the board due to space constraints, they can also be positioned along the board edges. However, it’s recommended to have fiducial marks near components with fine-pitched leads within the board as well. This enhances the accuracy of component placement.