In search of 4B3A Microswitch Keyswitches (for a restoration, not a keyboard collector!)

[Ian Finder]

Mistake below, Vo should be a fraction of 5 volts, not "a volt" when
operating

On Fri, Jul 10, 2020 at 2:28 PM Ian Finder <ian.finder at gmail.com> wrote:

> A few responses-
>
> On Fri, Jul 10, 2020 at 12:29 PM Brent Hilpert <bhilpert at shaw.ca> wrote:
>
>> Reading that datasheet, it appears that to reduce power consumption those
>> sensors employ pulsed sleep/wake operation, presenting up to a 1/8 second
>> delay in response time. Probably not very good for use in a keyboard, and
>> something to watch out for if looking for a modern replacement sensor.
>>
>
> Great catch, Brent. I'll keep looking at other options. I must have
> skimmed the data sheet a bit too hard.
>
> On Fri, Jul 10, 2020 at 01:19 PM Santo Nucifora wrote:
> > I can't help as I have a full keyboard that I'd like to keep intact but
> this is a home made...
>
> I have many of the keyboards with these 3-pin switches. As mentioned
> previously, what the sensors need to be electrically is the ***A sensor,
> which is harder to find by a considerable margin. Sink level, vs sink
> pulse, vs scan. In Al's words-  non-multiplexed contact closure type.
>
> http://telcontar.net/KBK/Micro_Switch/SD
>
> The machine is a Xerox system.
>
> On Fri, Jul 10, 2020 at 02:01 PM Al Kossow wrote:
> > It would be interesting to understand the failure mode.
> > I wonder if they passivated the die correctly
>
> I agree. What is interesting to me is this- If I look at other hall effect
> keyboards, and even other examples of the same sensor:
> http://telcontar.net/KBK/Micro_Switch/images/SD/Micro_Switch_SD_Hall_sensors.jpg
>
> They appear to be encased in a hard, black epoxy of some kind. Some of the
> other keyboards I have with the same SD switches- but alas the wrong sensor
> output type- appear to exhibit this as well, and have not failed.
>
> My failed sensors have something with the gelatinous consistency and
> transparency of RTV silicone. It did not appear to have become conductive,
> but who knows. There were three distinct failure modes:
>
> * 1) Output stuck at Vcc (+5v) - By the spec sheet, this should never
> happen. Vo should be a fraction of a volt when not actuated, dropping to 0v
> when actuated.
>
> * 2) Output stuck at 0v - Self explanatory.
>
> * 3) Output appears to be correct-ish Vo, but does not actuate to 0v with
> magnet. This was the rarest failure.
>
> Based on this, my best guess is that there are two failure modes:
>
> 1) RTV silicone decomposes over time, leaching something into the die that
> destroys the IC's passivization layer per Al's comment
>
> 2) The RTV silicone (or similar) is not robust enough to shield the
> delicate sensor die to ceramic substrate connections from vibration and
> force-based trauma as it decomposes.
>
> My expectation is that most of these switches have the black epoxy style
> of package for a reason- this is the correct design, hopefully should last
> a long time, and the ones that use this transparent goop will all
> inevitably die.
>
> I have never seen any other semiconductor die encased in what appears to
> be silicone...
>
> This is, incidentally, more about keyboards than I have ever cared to
> know. The fact that this is an entire hobby for some people is all the more
> shocking to me. I'd far rather be debugging some logic...
>
>
> Cheers- I
>
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