All,
I've been using through-hole and lately SMD devices extensively on one
and two layer boards, but I'm about to make my first foray into (what I
consider to be ) big BGA components and thats pushing me to 4 layers
(hopefully not higher) boards and more complexity.
So I'm looking for experienced viewpoints... since the excercies has
brought several questions up when I've been thinking about this...
- I have a 289 pin MAPBGA device (a freescale i.MX21 processor), that
I've created a footprint for , its a 0.8mm pitch part (there is a 0.65mm
version too... yikes), the pad layout seems pretty straightforward,
there is a 7x7 central cluster of pads, surrounded by a 2 pad space then
a 4 deep row of pads around the periphery of the chip.
- the recommendations for solder mask seem to be make the mask slightly
smaller than the pads (, balls = 30mil diameter, pads=30mil, mask=25mil)
to encourage good reflow of the solder balls.
- Next comes the routing and layout of the board..., can I create a
package with pads in a figure 8 shape (sometimes called dog-bone), one
end being the pad the other being a via to one of the four layers of my
board or a simple connection to this layer and out to the rest of the board.
- I think I can get away with a 4 layer board, 4 layers should be enough
for me to route all the pins I need, BUT... will the board houses allow
me to put a bunch of signals on the two internal layers that they seem
to expect are power supply and ground planes..., is this just
terminology/convention or is there a real limitation here. I'd be
bringing those signals up or down to the conventional two layers as soon
as practical but there won't be ONLY VCC and GND on those middle layers
- Most of these signals are fairly low frequency, apart from the LCD TFT
driver signals... so hopefully I won;t run into layout issues for
signa/noise problems.
- I really don't want to have to think about a 6 layer design since that
means a more expensive Eagle license and a more expensive board
manufacturing cost.. this is all hobby based stuff..
I'm calling for the voice of experience with this and any related issues
you've run into ....
