Why do PCBs need to be made in Panel?

From PCBworld,

01
Why puzzle
After the circuit board is designed, the SMT patch assembly line needs to be attached to the components. Each SMT processing factory will specify the most suitable size of the circuit board according to the processing requirements of the assembly line. For example, the size is too small or too large, and the assembly line is fixed. The tooling of the circuit board cannot be fixed. So the question is, what should we do if the size of our circuit board itself is smaller than the size specified by the factory? That is, we need to assemble the circuit board and put multiple circuit boards into one piece. Imposition can significantly improve efficiency for both high-speed placement machines and wave soldering.

02
Glossary
Before explaining how to operate in detail below, first explain a few key terms
Mark point: as shown in Figure 2.1,

It is used to help the optical positioning of the placement machine. There are at least two asymmetric reference points on the diagonal of the PCB board with the patch device. The reference points for the optical positioning of the entire PCB are generally at the corresponding position on the diagonal of the entire PCB; the optical positioning of the divided PCB The reference point is generally at the corresponding position on the diagonal of the sub-block PCB; for QFP (quad flat package) with lead pitch ≤0.5mm and BGA (ball grid array package) with ball pitch ≤0.8mm, in order to improve the precision of placement, it is required Set the reference point on the two opposite corners of the IC

Benchmark requirements:
a. The preferred shape of the reference point is a solid circle;
b. The size of the reference point is 1.0 +0.05mm in diameter
c. The reference point is placed within the effective PCB range, and the center distance is greater than 6mm from the edge of the board;
d. In order to ensure the recognition effect of printing and patching, there should be no other silk-screen marks, pads, V-grooves, stamp holes, PCB board gaps and wiring within 2mm near the edge of the fiducial mark;
e. The reference pad and solder mask are set correctly.
Taking into account the contrast between the color of the material and the environment, leave a non-soldering area 1 mm larger than the optical positioning reference symbol, and no characters are allowed. It is not required to design a metal protection ring outside the non-soldering area.