Quality control of PCB boards filled with SMD chips:

The production of electronics circuits creates a demand in new quality control instruments. When SMD chips are bonded onto printed circuit boards (PCB) several analysis tasks have to be fulfilled. A few examples are the measurements of:

• Misalignments of components
• Surface topography
• Dimensions, spacing,
• Contours, component angles
• Roughness of bondings
• Defects and cracks
• Electrical properties by probing
• Adhesive quality with force-distance curves

SMD chips have a wide range of outer dimensions and are placed nearby each other on the board. So the space of tactile measurement instruments or current probing systems is very limited. In addition the SMD chips have no sharp vertical walls, they get smaller in size near by their bottom line. This fact avoids to resolve details of the bonding area between SMD chip, adhesive and PCB. The following 3D-Nanofinger® measurements are one example out of several analysis tasks. In this case the SMD chips were fixed on PCB boards by micro adhesive bonding with a filled conductive adhesive.
To get an impression about the quality of the dispensed filled conductive adhesive a sample was prepared by micro adhesive bonding of single filled conductive adhesive dots onto a PCB. The PCB includes pure substrate areas, strip lines and contact pads:

PCB with pure substrate, two contact pads and the edge of an adhesive

The upper image shows a 3D-dataset that is scaled in all 3 directions. Already this overview picture offers the dimensions of the components as well as the contour of the edge areas and the surface roughness of the filled adhesive. Quantitative details can be gained by single Linescans through this area, as shown with the next example:

The next sample includes an additional small SMD chip (a capacitor) that is fixed on the filled adhesive. The transition between substrate, adhesive and SMD chip is hidden by the round shape of the SMD chip. Still it is possible to measure along this important area by using a tilted probe tip installed at the 3D-Nanofinger®. When the PCB is small and can be handled outside of an automation process it can be installed under a tilt angle of e.g. 45 degree in the 3D-Nanofinger®. Both options lead to the same result. The exact value of the selected angle is completely unimportant, because it will be removed later by software anyway:

3D image of SMD chip, adhesive, contact pad and PCB (from left to right)

Linescan through SMD chip, adhesive, contact pad, substrate

In the upper Linescan a filter function called "Sloping Level" is placed. Therefore two markers are placed with the mouse into the straight part of the substrate line. A straight line is calculated that forms the new baseline for the X-axis. The result of this coordinate transformation is shown in the lower diagram window. The substrate is now exactly horizontal, and in the left part of the Linescan discloses that the SMD chip indeed has a round shape like an undercut at its bottom end. This Linescan proves that the 3D-Nanofinger® can measure undercuts and resolve areas that are invisible from the top. The following image shows this Linescan including measures:

Linescan through SMD chip, adhesive, contact pad, substrate after coordinate transformation

Scale bars can be placed into the diagram to determine distances. In this way the height of the contact pads is determined to 56.3 microns. The adhesive has a thickness of 100 microns.