William "Chops" Westfield wrote:
> On Feb 28, 2011, at 1:11 PM, M. Adam Davis wrote:
>
>  =20
>> I'm comfortable working with LQFPs down to 0.4mm pitch
>>
>> The LPC1100 comes in an lqfp 48,
>>    =20
>
> I was looking at their "low pin count" LPC1102, which comes in a 16pin =20
> BGA (0.5mm pitch.)  What kind of design rules does one need to make a =20
> proper PCB for that kind of package?  Can you cheat, since there are =20
> only the four "internal" balls?  (except for them, it looks sorta like =20
> a 12pin LCC package, which I'd be willing to attempt.)
>  =20
The big issue is how to get the signals out from the internal balls. The=20
classic way to get signals out from the internal balls in a BGA is to go=20
out diagonally to a via  but with a 0.5mm pitch chip that would require=20
a TINY drill. Vias in pads is another option but IIRC those vias have to=20
be filled with copper to give a good soldering surface which adds to the=20
board costs. Coming out on the top layer doesn't look very attractive=20
either as the tracks would need to be tiny. I haven't managed to find=20
reccomendations on pad size but assuming a 0.35mm pad bringing out a=20
track between pads would require a 0.05mm (just under 2 mil) track size.

> For comparison, I usually try to make 0.5mm be my smallest drill...
>
> BillW
>
>  =20

--=20
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