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The right hand image is a flow sensor IC for an optical mouse available from Sparkfun. On the left is another optical flow IC with a camera attached from a Chinese distributor. It seems that the holes in the IC are a common property for optical flow sensor ICs.

What purpose do they serve? Why are they there?

enter image description here

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  • \$\begingroup\$ It does seem weird, my guess is for thermal reasons. these are passive sensors but still consume enough energy that perhaps they heat up. Normally PCB thermal pads are good enough, not sure why they bother with holes in the package. Could also be related to ambient pressure density of air? My final guess is for test-point needles to be inserted into the package post-production to run diagnostics/test sequence on the IC. \$\endgroup\$
    – KyranF
    Jul 29, 2015 at 17:50
  • \$\begingroup\$ Indeed a question I would like to know the answer myself,that optical sensor IC in my mouse has always astonished me,I always thought they were made for refractive and reflective properties of the IC and are specific to the design,but still I would like to hear other's view about it. \$\endgroup\$
    – MaMba
    Jul 29, 2015 at 17:55
  • \$\begingroup\$ They might be for optical alignment of the sensor with the IC. (Just a WAG) \$\endgroup\$ Jul 29, 2015 at 18:02
  • \$\begingroup\$ Approximately the number of pins +2 on each. Odd. \$\endgroup\$ Jul 29, 2015 at 20:10

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These chips were designed for optical mice and are part of a multi-part assembly. The outer rectangular holes are probably alignment features for the clip that holds the IC down during reflow soldering. The clip keeps the IC at the correct height and orientation as wells as providing a shield for the backlighting LED.

I talked to a friend of mine who designed some of the original optical mice chips for HP in the 90s. Since the die is mounted on the topside of the package, the lead frame is supported by pins during injection molding. When the mold and the pins are removed, the holes remain.

Data Sheet Ref, page 3 & 4. datasheet You can also look at datasheets from other Avago parts at Digikey.

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Almost certainly alignment pin holes from the injection moulding machine.

They will be used to keep the internal leadframe sections in the correct position when the casing is moulded so that the IC can be placed in the final place and bonded to the leads inside the case.

Most ICs are assembled on the leadframe while it still has metal bridging the pins and they are cut away after encapsulation moulding so the leadframe alignment is not disturbed during moulding.

These devices likely use a somewhat different moulding process as they have to keep the bonding pad locations free of plastic and in the right place to allow for wire bonding after IC attachment.

Another thought occured to me that may be equally relevant, they may be used to hold anvils (a bunch of tapered flat topped pins) during the ultrasonic wire bonding process so it makes a reliable bond when otherwise the plastic case would have to provide the counterforce which may be inadequate in practice.

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  • \$\begingroup\$ It is unlikely that those features are from ejector pins. Most ICs are rather featureless except for possibly an indentation to indicate pin 1. \$\endgroup\$ Jul 29, 2015 at 22:28
  • \$\begingroup\$ Ejector pins are used to remove the product from the mould cavity, you are correct that these pins would not be for that purpose. \$\endgroup\$
    – KalleMP
    Jul 29, 2015 at 22:34
  • \$\begingroup\$ It is important to remember that these optical devices are made without the IC in place because it is not encapsulated by the resin (plastic) and the metal lead frame has to be exposed. To prevent the lead frame from being covered with the resin it is clamped between two halves of the mould. \$\endgroup\$
    – KalleMP
    Feb 6, 2019 at 10:57
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I was very interested in this myself and I have taken apart about 10 sensors in the last few days and I am convinced the holes are used during the ultrasonic welding of the bond wires, as stabilization.

Because the sensor cant be covered with plastic like normal chips, they use partial plastic molding on the metal frame the sensor chip is placed on. Then to bond the chip to the metal frame they most likely insert pins in the holes to support the metal frame. The holes in the back are exactly lined up with the pads on the metal frame the chip is bonded to. This was the case on every sensor from every manufacturer.

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