Author: Wang Huidong, member of the YiBo Technology Expressway Team
"The design of this PCB optical positioning point is like autumn, which makes me feel refreshed."
Zhao Ligong, who was sighing about the spring and autumn, suddenly heard the discordant voice of the senior brother. He turned his head and asked with concern: "What's the matter, senior brother? You are so refreshed."
"Hey, Ligong Ruyan and other colleagues, come over together. Let's talk about the DFM case of this optical positioning point today." The senior brother invited everyone.
In an instant, everyone gathered around the senior brother and listened to his story.
What is the optical positioning point of PCB?
Scientific name: PCB optical positioning point
Alias: reference point, cursor point, mark point, etc. (more nicknames)
English name: fiducial mark
Function:
It is designed on the PCB and used for precise alignment and measurement during the manufacture of PCB bare boards. It is used for position identification points on automatic placement machines during PCBA, providing common measurable points for all steps in the surface mounting process, ensuring that the SMT equipment can accurately locate the components of the PCB board.
Comment: Don't underestimate this small optical positioning point, which plays a huge role on the PCB. It is willing to be the green leaves against the red flowers, hiding in the corner and silently devoting its beauty, and when illuminated by laser, it is stunning in an instant, and is known as the sapphire on PCB.
Classification of Mark Points
Based on the function of Mark points on PCB, they can be divided into panel Mark points, single board Mark points, and local Mark points (also called device-level Mark points).
Placement requirements:
There should be at least three Mark points on the process edge of the panel and on the PCB that does not need to be panelized, distributed in an L shape, and the diagonal Mark points are asymmetric about the center.
If there are components mounted on both sides, there should be Mark points on each side. Do not add only one side, otherwise the patch cannot be accurately aligned.
There should be Mark points on the PCB that needs to be panelized as much as possible. If there is no place to place the optical positioning point, you do not need to place the optical positioning point on the PCB, but you can add it on the process edge, remember.
For devices such as QFP with lead center distance ≤0.65 mm and BGA with center distance ≤0.8 mm, local mark points should be set at the diagonal near the diagonal of the center point of the component to facilitate accurate positioning.
If several identical IC devices are close together (≤100mm) to form an array, they can be considered as a whole and two local Mark points can be designed at their diagonal positions. The following situation is a bit too extravagant...
The shape of the point image feature of optical positioning
is a solid circle with a diameter of 1mm, and the material is copper. In order to increase the contrast between the mark point and the substrate, copper foil can be laid under the mark point. The inner background of the mark points on the same board should be the same, that is, the presence or absence of copper foil under the mark point should be consistent.
Surface treatment requirements: The mark point mark can be bare copper, bare copper protected by a transparent anti-oxidation coating, nickel plating or tin plating or solder coating. If solder mask is used, the mark point or its open area should not be covered.
Attention should be paid to its flatness, smooth and neat edges, and the color should be significantly different from the surrounding background color; the solder mask window is concentric with the mark point. The diameter of the solder mask window for the panel and single board can be 3mm, and the diameter of the solder mask window for the local mark point is 2mm. The forbidden area around the light spot should be larger than the size of the window...
Light spot placement area and requirements
The center of the mark point on the single board should be no less than 5mm from the board edge; the center of the mark point on the process edge should be no less than 3.5mm from the board edge. If the center distance from the light spot to the board edge is less than 3.5mm, the track will block the chip during placement, causing the machine to be unable to determine the coordinates normally and unable to perform normal placement.
When the panels are assembled, the 5mm wide craft edge and the light spot are added in the center. It is really for no other purpose except to look symmetrical. When the components are actually attached, the light spot is blocked by the track and cannot be recognized by the machine, so it cannot be accurately positioned.
The following figure shows the standard for adding light spots of our company. Please note that when adding 4 light spots, one must be foolproof. Otherwise, after the PCB board is rotated 180 degrees, the light spots can still be aligned, and there will be moths when mounting the chip...
The mark points of single boards and assembled boards should be designed as components, and the local mark points should be designed as part of the component package to facilitate the assignment of accurate coordinate values for positioning.
DFM case about light spots
1. There are SMD devices in the PCB, but no optical positioning points are added to the whole board, which results in the need to use small pads or holes in the board for positioning during mounting, resulting in poor chip offset and low production efficiency.
2. Because the size of the PCB board is too small, it is impossible to place the global light spot inside the board. In order to place the SMD assembly position later, the light spot is added on the process edge, and it is added on the center line of the V-cut. Then, during the molding, there is a "click" sound, and the two light spots are broken, and they are instantly wiped out... This is a case that makes the senior brother shout "autumn is cool and refreshing"...
In order to ensure the recognition effect of printing and patch, there should be no pads, vias, test points, traces and silk screen logos within the range of the Mark point, and they should not be cut by the V-CUT slot, causing the machine to be unable to recognize them. The cleanliness of the light spot forbidden area must be ensured, otherwise if the equipment recognition sensitivity is not high, frequent shutdowns will occur during placement.
3. If the distance between the device and the board edge is less than 5mm, you need to add process edges and light spots, otherwise you need to use a fixture. What about the cost-down? Money...
When Lin Ruyan heard the senior brother talking about the cleanliness of the light spot, she suddenly became inexplicably sensitive. She suddenly thought of a problem and wanted to go back to her computer to check. She was afraid that there was a problem because a BGA had a local light spot. In order to make room for wiring, she deleted the forbidden area of the light spot. There was space for routing, but if there was something in the forbidden area, the cleanliness could not be guaranteed. She quietly returned to her seat and clicked on the screen. Oh my god, what she feared really came true. There was really a routing in the clear area of the light spot...
"Fortunately, I saw it myself, and others didn't see it. Change it..." Lin Ruyan said to herself while carefully changing the forbidden area and routing of the light spot.
"Who said others didn't see it? I saw it. Hehe..." A voice sounded in Ruyan's ear at an inopportune time.
Lin Ruyan looked up and saw the gloating smile of Zhao Ligong.
"What did you see, Zhao Ligong, do you want to wear a watch again?" Lin Ruyan stared at her with her beautiful eyes and gritted her teeth and said angrily.
"I saw a beautiful woman, how can I not do it?"
"Hateful," Lin Ruyan looked at her and laughed.
"It's getting late, and everyone still has to do the design. The case ends here, and the light spot case will continue next time..." said the eldest brother.
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