[Arm-netbook] laptop 15in pcb1 progress

[Luke Kenneth Casson Leighton]

On Tue, Sep 29, 2015 at 11:39 AM, joem <joem at martindale-electric.co.uk> wrote:
>
>>  so i'm using the T862A as a PCB development tool
>
> No. Nurse the oven as your tool.

 remember, joe, you've done this a _lot_, so have much more confidence
in your abilities, as well as, from experience, a much higher
anticipated success rate.  i had no idea what i was doing, so i needed
something which i could monitor closely and much quicker (like...
immediate), on a much smaller area.

 so: i can't see inside the oven with the door closed, can i?  that's
what i like about the one with the lamp: it focusses the IR on a small
area (about 5cm in diameter), i can get tweezers in underneath it and
remove components (or adjust their positions), i can watch the solder
actually melting, i can actually *see* the point when the components
flow into the solder, i can get solder in with a bit of solder paste
on the end to encourage the solder on the board to melt...

 pluuuus, i can put only a tiny amount of solder paste down, which is
tolerable with the windows open.

 you can't do _any_ of those things with an oven: it heats up the
entire board, it's an enclosed space, you have to heat up *all* the
components including all the ones that you previously placed, it takes
a long time to warm up and cool down...

 so you *have* to do all the components (or risk re-heating all of
them).  whereas i'm safely doing 5-10 components at a time, confident
that because the tested ones are in another area away from the lamp,
they won't get moved, overheated or (in the case of the TSSOP-56)
warped beyond useability.

 when all the components i've put down are clearly visibly in full
solder-contact with the board, i can shut down the lamp immediately.
sometimes i've managed to get components soldered in about... 4 to 5
minutes.   removing components (including SOT-23s and TSOP-8s) can be
as little as 3-4 minutes.   can you do that with an oven?  no.... you
can't see inside it, clearly, so you have to play it safe and let it
heat up on a full cycle, which is what... 10 to 15 minutes at a time?

you said it yourself: you destroyed several boards, first time you
used the oven.  i only destroyed one, and it was the first test board
i'd ever done, it was only 1in x 0.5in in size (the fibreglass
actually delaminated).  i learned from that, and haven't damaged the
laptop PCB at all.  i've only destroyed around 3 components in total,
one was the TSSOP-56, it actually curled up at one end due to uneven
heat distribution and i wasn't watching the lamp's temperature sensor
closely enough.  so i'm leaving its replacement until last.

 now, the down-side is: it's taken 2-3 weeks of about 1 to 2 hours a
day to put down all the components... and there's only about 120 maybe
150 components.  that's *ridiculously* slow.

 ...but i don't care if it's slow: this is my first PCB since..
since.... making illegal FM transmitters back in 1983 from "the
anarchist's handbook".  i've done the "pay someone else to design
PCBs" thing - that didn't work out.  even paying someone else to
assemble PCBs is relatively pricey (if you get it wrong more than zero
times).

 so i am simplifying the task to one which, with my skill set, stands
an above-average chance of success and/or is a much lower iterative
cost.


>  Its more safer and use as many times as
> you want. (Some items such as LEDs won't survive re-use too many times -
> but they are cheap to replace.)

 found that out already :)  one of the other components i destroyed
was an 0805 LED - i hand-soldered the next one...

l.