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Zulki's PCB Nuggets
By Zulki Khan
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Zulki’s PCB Nuggets: PCB Microelectronics—Inspection and Calibration
Microelectronics manufacturing is the companion of SMT manufacturing and forms PCB hybrid manufacturing. Tools for SMT manufacturing have been around for a long time and have proven their value. Now, with microelectronics, new and different types of high-powered laser microscopes are populating the microelectronics assembly and manufacturing area to provide highly effective inspection and calibration.
Scopes like the Keyence VK-X 1000 are giving microelectronics customers assurances that substrates and dies are intact during incoming inspection as well as post-assembly verification (Figure 1). Plus, they are precisely calculating measurements. They are also taking highly accurate readings for the specific height, length, and width of wire bond loops and curvatures to make sure specifications are met, as defined in the scope of work.
Figure 1: High-powered laser microscope.
Inspection tasks include die, epoxy resin and solder mask bleeding, and air bridge. Calculations include length, width, and height in the Z-axis. Also, these scopes create 3D rendering, making the images more vivid.
Microscopes inspect the die surface for defects. Those can be surface anomalies, any type of damage—such as fine cracks, tiny die corner chipping, corrosion, contamination, or oxidation. In this case, die inspection is expected to meet MIL-STD 883 Revision G or E.
The possibility of epoxy resin and solder mask bleeding is inspected as well. At times, misjudging the amount of epoxy under the die for die attach leads to a floating die. That is definitely not desirable. The high-powered laser microscope verifies that the die is 100% on a substrate’s or interposer’s base, creating perfect joints.
As for solder mask bleeding, the mask may bleed onto the pad where wire bonding is to be installed. The issue is the pad’s surface may not be sufficient in size to perform the bonding. Again, these scopes inspect and verify this issue doesn’t exist.
Other times, flux or epoxy residues may create similar issues. If PCB hybrid manufacturing—meaning a combination of SMT and microelectronics manufacturing—is being performed, some residues may be leftover from SMT manufacturing. Those residues may remain on a PCB’s substrate or creep into areas where microelectronic components are to be placed. As a result, the surface becomes uneven or unfit for die attach or wire bonding. When this occurs, the bond to be created at the wire bond pad becomes difficult if not impossible.
Therefore, cleanliness is of the utmost essence. The thinner the wire bond is, the more precise the pad needs to be. Typically, thin wires used for the pad are extremely close to one another. If the thinnest or finest 7/10 of a mil wire is used, the surface needs to be as clean as possible by applying either alcohol, isopropyl alcohol (IPA), or argon gas for extra cleanliness.
They also inspect air bridges. An air bridge is the air distance created to bypass a component located between two other components. That’s done to connect a wire bond from one point to another and overpassing the middle component.
Moreover, aside from inspections, these microscopes calculate length, width, and height in the Z-axis for dealing with height restrictions of dies and substrates. Sometimes, dies are attached in an ENIG or ENEPIG surface finish-based cavity. Dies need to be very precisely height controlled. This permits perfect fit in those cavities before wires can be attached via wire bonding after die attach. The high-powered scopes are perfect tools to view cavity depth as well as to perform height measurements in the Z-axis.
Finally, 3D rendering of the pads, substrate height, or paste height provide microelectronics manufacturing technicians a visual. This allows them to calculate the distance between the substrate and the wire bonding as well as check the thickness of the epoxy or the solder paste that is being used.
It is absolutely essential to calculate intricate fine wire bond loops and curvatures for some projects. For example, a 3D profile is created for a wire bond loop’s 5-µm accuracy in measuring the height. In this 3D profile, various aspects can be viewed. Those include die and epoxy thickness and distance between the substrate or interposer and die. Creating this 3D profile gives a very clear-cut picture, which can easily be precisely measured. It also provides a very good visual, which is used as a proof of effective versus poor microelectronics assembly.
In summary, there are certain PCB manufacturing aspects that even the finest X-ray devices cannot look at precisely, whereas these high-powered inspection tools come into play to deal with intricate details to perform perfect microelectronic assembly.
Zulki Khan is the president and founder of NexLogic Technologies Inc.
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