Маркировочные точки при проектировании печатных плат
Mark points, also known as optical points or reference points, are crucial for identifying positions on circuit boards during the assembly of components. Their selection directly impacts the efficiency of automatic placement machines, necessitating careful design considerations for Mark points and their locations on the board.
Design of Mark Points
1. Layout Position
Single Board Mark Points
When designing PCBs, Mark points should be added to the side that requires component placement. For double-sided placement, Mark points should be added to both sides, ideally at all four corners in an asymmetrical arrangement to prevent mismatch. If space is limited, three Mark points may suffice; however, at least two should be added diagonally if necessary.
Panel Mark Points
Panels require Mark points as well, with points at each of the four corners of the processing edge, avoiding symmetrical placement for foolproofing. If the panel does not have a processing edge, Mark points should be added within the board itself. When no Mark points are added to a single piece, at least three should be placed in the empty areas of the connected panels.
Component Mark Points
To enhance the placement accuracy of certain components, such as QFPs and BGAs, dedicated Mark points should be added, positioned at two corners diagonally opposite each other on the component.
2. Design Specifications
Shape and Size
Mark points can be circular or square, typically sized at 1.0mm, with the solder mask opening being 2.0mm. To avoid glare affecting Mark point detection, the opening must exceed the line PAD by more than 0.5mm. In constrained spaces, an opening of 1.5mm is permissible, but the line PAD must exceed 1.0mm.
Edge Distance
The safety distance of Mark points from the board edge should exceed 3.5mm to prevent obstruction by the conveyor during processing. If located on the processing edge, they can be adjusted inward, ensuring that internal Mark points remain at least 3.5mm from the edge.
Open Areas
Avoid placing traces or components near Mark points as this affects recognition. A clearance area of over 3mm should be established around each Mark point to ensure optimal accuracy for SMT placement equipment.
Application of Mark Points in SMT
Principle of Mark Point Usage
During the placement process, misalignment can occur. Mark point positioning effectively addresses this issue by serving as a reference for location. Placement machines equipped with Mark point functionality can better determine the accurate placement of components, ensuring high placement precision.
Methods for Placing Without Mark Points
In the absence of Mark points for automatic placement, a specific solder pad can be selected as a Mark point. Taping a stencil or creating a fixture with a Mark point can serve as an alternative method; however, precision will likely be compromised without traditional Mark points.
Production Cases Without Mark Points
Problem Description
The absence of Mark points led to misidentification of points, causing components to be incorrectly placed.
Impact of the Problem
Incorrect placement resulted in the loss of multiple components, delaying product development and wasting R&D and manufacturing costs.
Problem Extension
Improper placement of components due to the lack of Mark points may hinder product functionality, necessitating re-production and re-sourcing of components for assembly.
Mark Point Inspection Solutions
Utilizing Huachiu’s DFM software to inspect Mark points can prevent losses in product development due to missing or improperly designed Mark points. Checking design files prior to design can reduce R&D costs and production cycles.
Part.1
Huachiu’s DFM software detects whether design files are missing Mark points. If absent, the software will indicate the need for Mark points to serve as positioning references for placement machines, allowing for corrected coordinates and precise placement on PCB locations.
Part.2
Huachiu’s DFM software can also identify abnormalities within Mark point designs. Checks include the size of the Mark point opening, the safe distance to edges, and the influence of surrounding objects on Mark point recognition. This software can fulfill a variety of design inspection requirements.
