In microcontroller development, we often need to hand over the designed PCB files to the factory for production. Recently, one of our engineers encountered a troubling issue, as the PCB board lacked Mark points, resulting in deductions from the SMT factory. Today, we will discuss this topic: how important are Mark points in SMT processing? Is it really impossible to perform SMT without Mark points?
1. What exactly are Mark points?
If you drive in an unfamiliar city without road signs, what might happen? Mark points are like road signs for the pick-and-place machine, just like the positioning coordinates on a PCB, typically designed as circular pads with a diameter of 1mm. The pick-and-place machine identifies these points to automatically calibrate the position of the PCB, similar to how a GPS accurately locates:
Compensating for PCB manufacturing errors (±0.1mm), correcting the positional deviation of the PCB on the fixture, ensuring high-precision components, such as BGA, QFP, achieving consistent positioning during mass production.
2. Can SMT be performed without Mark points?
The answer is: yes, but with conditions and risks:
For small batch samples, for example, <10 pieces, skilled workers can manually position them. For simple boards, such as development boards with only through-hole components, the SMT factory can use the board edges or through holes for positioning, serving as alternative reference points.
In these cases, Mark points must be used:
For high-density boards with component spacing <0.3mm. For complex boards, BGA packages, 0402 small components, and other precision devices like QFN, and for orders exceeding 50 pieces. For special processes, requiring double-sided mounting of PCBs.
3. What problems can arise from the lack of Mark points?
Decreased placement accuracy: offset errors may exceed 0.2mm. Reduced production efficiency: debugging time will increase. Lower yield: small components below 0402 are prone to poor soldering. Increased rework costs: manual re-soldering may be required.
4. Why can some factories handle boards without Mark points?
This mainly depends on the factory’s hardware configuration:
Whether the vision system supports feature point recognition. Whether it is equipped with high-precision mechanical positioning devices. Whether it can accept manual intervention adjustments. However, it is important to note:
even if SMT can be performed, there are still risks for high-precision components like BGA, QFN, etc.
5. Points engineers need to pay attention to:
Design specifications: It is recommended to add 1mm diameter Mark points at the four corners and center of the PCB.
File annotations: Clearly require the retention of all Mark points in the board manufacturing instructions.
Communication confirmation: Confirm in advance with the factory whether they automatically add Mark points, or request process requirement documents.
Contingency plans: If boards without Mark points have already been produced, negotiate whether specific component pads can be used as alternative reference points.
Mark points are like road signs for the pick-and-place machine; without them, the machine can easily get lost. Although some experienced manufacturers can barely use the board edges or pads as substitutes, it is like trying to find a location with a blurry map; the placement accuracy will be compromised, small components are more likely to be misaligned, and production speed will slow down. Therefore, it is essential to have Mark points. This is akin to laying a foundation when building a house; it may seem insignificant, but without it, the entire project could go awry. Experience: If your PCB prototyping and SMT processing are done at the same factory, the factory will automatically handle the issue of Mark points, as this is a necessary condition for ensuring high-quality processing. If PCB prototyping and SMT processing are done at different manufacturers, it is crucial to pay attention to the issue of Mark points, as the PCB has already been prototyped, and Mark points cannot be added, which may lead to additional costs.