Catching seating defects on 1 mm IC packages that 2D vision missed: Inspection using KEYENCE Pattern Projection Lighting

The machine automates the loading and unloading of a reflow oven for a semiconductor manufacturer that had been doing it by hand — time-consuming, labour-intensive, and costly. Inventit studied the manual process and proposed automating the whole sequence: transfer the units from a JEDEC tray to a reflow tray, pass them through the reflow oven, then transfer them back from the reflow tray to a JEDEC tray. The trays are stacked one on top of the other and flipped 180 degrees to move the units across. The system inspects at three stations: seatedness before the flip, count after the flip to confirm nothing was dropped, and count plus seatedness again after reflow, before unloading. The end customer runs up to about 30 different package types, from 5 mm by 5 mm to 15 mm by 15 mm, with a recipe change at every size changeover.

Inventit itself builds automation machines that are not available off the shelf, customised to each customer’s requirements. Its work is concentrated in the hard disk drive media and solid-state memory industries, where much of the equipment is standard but not connected to anything else, so a large part of what Inventit builds is the material handling and transport that links those standalone processes together, by conveyor or by AGV.

"Many of these processes in hard disk drive have standard equipment, but they do not integrate with each other. So what we do is link up all the machines together, either through a conveyor system, or AGV nowadays, to transport the material from one process to another."

In the solid-state memory field, though, much of the work involves customised tooling that is part of the assembly process itself — checking a part before or after it is assembled — rather than just moving material between machines. That is usually where vision inspection becomes necessary.

Because the trays are stacked and flipped, seating is a safety-critical condition rather than a cosmetic one: if a chip is not sitting properly in its pocket, the tray placed on top of it will damage the unit, and at about 1 mm thick the packages tolerate very little force. The inspection was originally built around 2D imaging, and that is where the difficulty began. A tray pocket has depth, and a 2D camera sees only X and Y from above. A package tilted slightly inside its pocket looks much the same as one lying flat, so 2D imaging cannot judge seating reliably unless the package is completely out of the pocket. Inventit puts the escape rate at roughly 40 to 50% of the seating defects.

"The units are only about 1 mm thickness. So a slight tilt inside the pocket, from a 2D perspective, cannot tell that it's actually seated correctly or not — unless the unit is totally out of the pocket. That's why we were facing a very high failure rate."

One alternative was on the table: a 3D line-scan laser that plots a 3D image of the tray. The end customer suggested it, and a supplier known to Inventit had already implemented it. Technically it would have worked, but a line scanner has to travel the length of the tray to build up that image, and it therefore needs a large installation envelope. The machine had already been designed, manufactured and delivered; the only space available was the space that had been allocated to the 2D camera. Pattern projection lighting for the KEYENCE XG-X Series was launched in Singapore at almost exactly that moment, and because it needs about the same space as a 2D camera, it was the one option that could be fitted to the machine as built. KEYENCE had been involved in the original 2D inspection and stayed with the project while the alternatives were being weighed.

"Using a line scan, it requires a big space to implement this application, because the scanner has to move along the tray to scan the whole tray. But at that time we had already delivered and manufactured this machine, so space was something that we could not afford at that moment. The pattern projection needed a similar space to the 2D vision. That's why it was the best option that we had at that time."

Inspection using pattern projection lighting captures height in addition to position, which is precisely what the application had been missing. Detection of seating defects went from the 40 to 50% escape rate of the 2D setup to about 95% — the figure the end customer had specified. Inventit attributes the remaining margin not to the vision system but to normal variation in the trays and in the units themselves. Cycle time was not compromised: pattern projection takes around 4 to 5 seconds against about a second for the earlier 2D check, but inspection was never the bottleneck, so the machine still meets its requirement of 25 seconds per tray. The clearest change on the floor was in human intervention. To keep production going while the inspection problem was open, the end customer had posted one operator at the front of the machine and one at the back to watch every transfer and catch damage. After the retrofit, that monitoring stopped; operators load and unload material and nothing more.

"The 2D actually had about 40 to 50% failure. So after introducing this, it improves to like 95%. Of course there will still be 5%, but not because of the vision — basically because the product has variation. In general it meets what the requirement or the specification the customer gave us, which is 95%."

Since that first project around 2017, pattern projection has been applied to other machines, including semi-automatic cells that run many different products. Those machines change nests at every product changeover, and each nest used to need a set of sensors to check presence, orientation and whether the operator had seated the part correctly. Now one camera checks all of it — presence, orientation, seating, and even whether the correct part has been loaded into the nest — which takes a great deal of complexity out of the mechanical design. Asked what he would tell another machine builder facing a similar inspection problem, Mr. Chua's answer was three words long.

"Approach KEYENCE first. When we have a problem, or a challenge that the customer brings to us, we need to partner with a company like KEYENCE to work closely with us until we can give a good solution to the customer. With the knowledge of the salespeople and the range of products that KEYENCE has, I think it's a very good partnership."