PCB Tech - PCB design: Why is the reflective point important?

In the field of PCBA technology manufacturing, this term may not be heard by everyone. However, as long as the suffix "ry" of the trust is replaced with "l", it becomes the fiducial that electronic engineers must know!

The function of the reflective point

If the circuit board is assembled with a pick and place machinery that can automatically install components on the circuit board, fiducial marks will come in handy. The accuracy of the placement machine must be very high to accurately place 0201 resistors or miniature BGAs with a 2mm x 2mm chip size. In addition, the machine also needs to know exactly where the circuit board is.

The reflective spots are specially designed for surface mount soldering machines. These marks can ensure that the circuit board is oriented correctly and placed as accurately as possible. The placement machine uses a camera to locate the reflective point, and then adjusts the position of the internal components based on the exact position of the circuit board. The entire process usually requires more than one irreversible reflective point.

When should I use reflective dots?

PCBA mass production and assembly work must be assisted by reflective points to ensure accurate registration and component placement. PCBA small-scale assembly such as small-scale production or prototyping is not necessary, but there are some exceptions, so you'd better confirm in advance. Even if you don’t need reflective dots to assemble a particular product, reflective dots are still useful, so it’s always a good thing, unless you are sure to rely entirely on manual component placement.

Design reflective points

When it comes to the special structure of the reflective point, two things are particularly important: location accuracy and contrast.

Location accuracy

The reflective point must be on the top copper layer. Why must a copper layer be used instead of screen printing or drilling? The answer lies in registration! The top copper layer is placed in one time, so the position of the surface mount pad relative to the reflective point of the copper foil will never change. In contrast, screen printing or drilling requires one more procedure, so that the registration of each circuit board It may be different.

Generally, the pad size of 0.4mm pitch BGA is 0.254mm (.01”), and the spacing between the pads is 0.15mm (.0059”). Screen printing and drilling cannot achieve the accuracy required for registration.

Contrast

The camera of the placement machine needs to receive a strong contrast, so the copper layer must not be bare copper. The solder mask on the copper pad may reduce the contrast, making it difficult for the camera to recognize. The solder mask has the same registration problem as screen printing, so you must ensure that the copper pads are clearly visible.

The diameter of the copper pad is about 1 to 2 mm, so the opening of the solder mask is 2 to 5 mm larger than that of the copper foil. Some CAD software suites have reflective spots in their component libraries, which is the easiest way.

The shape of the reflective spot

A close-up of the reflective spot on the circuit board. The solder mask opening of this reflective spot is square, generally round, but the shape of the opening is not that important. But the reflective copper pad must be round!

If your PCB circuit board comes from the connecting board, you can put the reflective point on the guide rail of the connecting board. If you are assembling an individual circuit board, you can make the reflective point directly on the board. If you are not sure, you can do both so that you don't have to worry about whether you want to assemble on even boards or separate boards.

Don't forget that reflective points are not necessary for small PCB manufacturing, but it is still worth doing even if it is not useful. The quality of electronic products is mostly related to risk management, so it is very important to design the circuit board well!