Solder paste application quality control (SPI) has in the past taken a lower priority than automatic solder joint inspection (AOI) systems. While the AOI solder joint system detects defects only at the end of the production line, the inspection of the solder paste immediately after its application makes a significant contribution to the improvement of the technological process. Although the inspection of finished products is inevitable anyway, the main goal of every manufacturer is to produce a quality product without the cost of repairing it. To achieve this result, it is necessary to control and manage each stage of the production process. Poor application of solder paste is often the root cause of defects that will appear in subsequent stages of production. The soldering process itself does not cause this defect, but only manifests it.
Benefits of 3D measurement in application control systems
Is random control of solder paste application sufficient?
Today there are product lines that include a printer with built-in 2D solder paste inspection and a standalone SPI. However, to maintain the speed of the printer, the built-in inspection system only inspects a few areas of solder paste application; The stand-alone control section only performs random checks, which cannot guarantee the absence of a defect on each board.
Only checking the entire surface of the board will reveal random defects, and only inspection of the entire series (each board) will reveal deviations in the operation of the printer. For example, a trend towards a decrease in the amount of solder paste applied over the entire series indicates fouling of the apertures.
Influence of Aperture Size on Imprint Efficiency
As electronic assemblies get smaller and the mounting becomes more compact, the size of stencil apertures decreases and their number steadily increases. Different sizes of stencil apertures affect the so-called transfer coefficient. The smaller the hole size, the less solder paste is transferred from the stencil apertures to the PCB.
Let us compare two round holes of the same stencil with a diameter of 700 and 270 µm with the same stencil thickness of 100 µm and evaluate the efficiency of the imprint.
It is easy to see that the application area varies from 75 to 100%, and the volume in all cases is more than 90%, which is a completely satisfactory result. For an aperture with a diameter of 270 μm, the picture is somewhat different. The application area ranges from 60 to 120%, and the volume – from 50 to 100%. The implication of these measurements is a priori that at small apertures, quality control of the paste application becomes a key factor in ensuring the quality of the solder joint.
2D or 3D inspection?
SPI vendors offer 2D and 3D solder paste control systems. 2D inspections work by comparing different shades of gray. This requires a lot of adjustment, depending on the color of the printed circuit boards, and does not provide accurate volume information.
3D systems are based on PCB and solder paste inspection. Two technologies can work in 3D inspection systems today – a laser measurement method and a multi-frequency method. Laser measurement reveals many defects that can be missed during a normal 2D inspection. But this technique has a number of limitations, for example, the inability to measure the volume of the indicated area, the measurement error due to the large thickness of the laser beam, and the sensitivity of the laser to the color of printed circuit boards.
Multi-frequency moire method
The essence of the multi-frequency moire method is to measure the height of each pixel of the inspected object. The measurement principle: light directed at an angle to the object from several projections resembles a diffraction grating. Due to the offset of each grating, we get the lines projected on our product. If the surface has any unevenness or relief, then the lines of the projected grating will shift.
The multi-frequency moiré technology makes it possible to measure the height of an object at each point (or in each pixel) with an accuracy of 1 micron by the shift of the fringes. Knowing the height at each point of the inspection area, we can determine the area of the object, and knowing the height and area, we can get the volume of the inspected object. It follows from all this that, unlike classical systems, 3D measurement using the multi-frequency moiré method allows not only to distinguish the volume (to determine the presence or absence of an object), but to obtain mathematically accurate measured data on the volume in numbers.