In my opinion, orienting IC’s so that their power pin is closer to another IC power pin is trouble. We’ll be sure to get at least one in backwards. Further, it’s not the proximity of a cap to a power pin that has the best results. It’s the proximity of the cap to BOTH power and ground that makes it most effective. I have seen sockets that embed an axial lead cap (most likely 0.1 or 0.047 uF) right into the middle of the socket with the shortest possible lead length for the cap to the power pins of the socket. Never used them, but I would think this to be quite effective, saves time installing bypass caps, saves board real-estate and saves some copper. However, the drawback is these sockets only work for ICs with “standard” power pins, they are (were) considerably more expensive and I have not seen them for sale for many years.
When I design a board, I try to create a very low impedance power supply (runs a wide and as short as practical, and ground planes wherever possible) and I bypass in a grid/network arrangement. My rule of thumb has always been one bypass cap for every 2 to 4 SSI chips, one bypass cap for every 1 to 2 MSI chips, and one bypass cap for everything else. Since CMOS does not create current spikes like TTL, the rules are a bit more relaxed for that technology, but even CMOS LSI I still bypass one cap for each chip. I think erring on the too many side is better than under-bypassing.
My 2 pence.
Bob Bell
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of Don Caprio
Sent: Thursday, January 01, 2015 2:15 PM
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:5946] SOIC Packages For S100 Boards - Survey Question
Thanks for all the feedback. There were a lot of good arguments
for both sides. Each with it's own merit.
I think the consensus is stick with through-hole. I have to admit
that I'd hate to give up sockets. This is a good way to debug a
board and I'd hate to give that up. Plus I'm getting up there in age
as well and my eyes aren't what they use to be.
For my next version of the mini I do have some ideas to make
make more real estate available. The filer caps take up a lot of room.
I could go with a smaller footprint. I'm also wondering if one filter cap
could be used for 2, or 4 IC's. It's nice to have all the Pin 1's oriented
the same so that would blow that standard.
I've also seen the filter caps put in the center of the socket. I'm not fond of this idea.
Replace resistor nets with discrete and place resistor between LED's (yuck).
--
Don Caprio
ilv...@gmail.com
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