On Sat, 16 Aug 2014 11:12:01 -0400, you wrote:
>
>A little out of my league <smile>. My needs are simpler, though I
>often do use TQFP packages. Those, however, often need vias under the
>chip, which I can't do. To make alignment less critical I would use
>larger vias so you can't "miss" as easily.
Well....
I don't do vias under the chip at all. I've recently hit on a design
that takes all the supply voltages, spiders them out a little, then
goes to the bottom layer. Under the chip are all of the bypass caps,
which have a big common ground plane under the chip itself. The chip
ground leads themselves go to a common ground plane under the chip on
the top layer. The ground leads from the chip are also spidered out
and run under the chip to the ground plane there. Seems a slight bit
complicated, but other than the vias required to go top to bottom, you
have pretty decent access to all the pins.
The latest vias have a 0.021 hole, and are about 0.043 in diameter, so
alignment can be a bit picky here.
Why so small (relatively, I guess)? 0.5 mm spacing TQFP 144 pin
packages is why...
Harvey
>
>Mark
>
>
>At 09:38 AM 8/16/2014, you wrote:
>>On Sat, 16 Aug 2014 08:51:49 -0400, you wrote:
>>
>> >Thanks, Harvey, for the detailed description - Very interesting
>> >setup. One of these days I'll work on something similar, though I
>> >rarely drill a lot of holes. I use almost exclusively smt and usually
>> >make single sided pcbs using zero ohm resistors as jumpers.
>> >
>>
>>I do double sided boards, with the boards made from two .021 boards
>>epoxied together (use 1 hour epoxy!). Alignment is very critical and
>>the drilling setup (finally!) makes that work a lot better. Since I'm
>>doing 64/100/144 pin TQFP chips, I can't live with the luxury of
>>exclusively resistive jumpers.
>>
>>Latest project involves a graphics panel driver with an S1D13781
>>graphics chip (Epson, anyone wanting to use it please talk to me,
>>there are some non-obvious things you need to do to make it work). an
>>Xmega, a touch panel controller, -24 volt supply, I2C communications
>>(dual channel).
>>
>>Harvey
>>
>> >Mark