Mark points, also known as optical marks or reference points, are critical for component assembly in PCBs, particularly in the context of PCBA (Printed Circuit Board Assembly) for automatic placement machines. The choice and placement of Mark points directly impact the efficiency of the automatic pick-and-place machine, so it’s essential to carefully design these points and their positions within the board.
Mark Point Design
Single-Sided PCB Mark Points
When designing a PCB, Mark points should be added on the side that will be populated by components. For double-sided assembly, Mark points should be added on both sides. Typically, Mark points are placed at the four corners of the PCB, ensuring the positions are asymmetrical to prevent incorrect usage. If space is limited, at least three Mark points should be added, and if the design is very tight, at least two Mark points should be placed diagonally across from each other.
Nesting Mark Points
Mark points are required for PCB panelization. If there are process edges in the panel, Mark points should be placed at the four corners of the process edge, ensuring the positions are asymmetrical and offset to prevent incorrect usage. If no process edge is present in the panel, Mark points should be added within the PCB. If the single PCB design does not include Mark points, at least three Mark points should be added to the blank area of the panel.
Component-Specific Mark Points
To improve the placement accuracy of certain components, such as QFPs, BGAs, and other complex packages, additional Mark points should be placed at the diagonal corners of the component’s footprint.
Design Specifications for Mark Points
Shape and Size
Mark points typically have a circular or square shape, and their diameter is generally 1.0mm. The solder mask opening for Mark points should be 2.0mm. Since solder mask ink can reflect light and interfere with the recognition of Mark points, the solder mask opening should be at least 0.5mm larger than the pad. If space is tight, a 1.5mm solder mask opening can be used, but the pad size must be at least 1.0mm.
Edge Distance
The minimum safe distance from the Mark point to the board edge is typically 3.5mm. This ensures that the pick-and-place machine’s rails do not obstruct the Mark point during the process. If the Mark point is near the process edge, it can be slightly offset inward. However, the distance from the Mark point to the board edge should generally exceed 3.5mm.
Clear Area Around Mark Points
Mark points should ideally be placed in areas free from traces or components, as nearby traces or pads can interfere with the Mark point recognition process. The area around the Mark point should be kept clear, and a minimum 3mm clearance should be maintained from surrounding pads or milling areas.
Application of Mark Points in SMT
Mark Point Usage Principle
During the component placement process, misalignment can occur. Mark points are used to solve this problem by providing a reference for locating the components accurately. Pick-and-place machines with Mark point recognition capabilities can better determine the placement positions of components, thus improving placement accuracy and ensuring that the components are placed correctly on the PCB.
Placement Without Mark Points
In the absence of Mark points, the only solution is to select a specific placement pad to serve as the Mark point. This involves using adhesive tape to attach a stencil and marking the point manually. In cases where this is not feasible, a fixture may be used, and the Mark point is added to the fixture. Although it is possible to perform component placement without Mark points, the placement accuracy is often poor.
Case Study: Lack of Mark Points in Production
Problem Description
The absence of Mark points led to incorrect identification of the Mark points during production, causing misplacement of components.
Impact of the Problem
The incorrect placement of components resulted in the loss of many parts, which delayed the product development process. Additionally, it wasted both R&D costs and manufacturing costs related to assembly.
Problem Extension
Without Mark points, the incorrect placement of components could cause components to be attached to the PCB incorrectly. This might result in a non-functional product that requires the board to be re-manufactured, with new components being purchased and re-assembled, which could add significant delays and costs to the project.
