It appears from the disassembled 35S photos That HP is using chip face down assembly. When I look at the circuit traces I see some dissapear without a plated thruogh hole. Can someone enlighten me as to there chip assembly method. Ultrasonic? Are there also bond wires? It seems to have too many PC leads to connect them all in one bonding. What happens when the traces seem to disappear?
Thanks, Sam Levy


There is nothing unusual at all here. The chip is most likely mounted using COB (Chip On Board) bonding technology covered with the black potting compound, a very common technique to lower cost in higher volume products.

The "vanishing" tracks are just an illusion caused by light reflection off the gloss black solder mask.

Interesting that they have chosen to use a hand soldered watch crystal rather than a machine assembled surface mount package, they probably saved a cent or two here, but it requires two extra human operations (solder+red blob)


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