Electronics > Projects, Designs, and Technical Stuff
Is it good design to put SMT components beneath socketed DIP packages?
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Benta:
Technix, you should take look at the old PEP Modular Computers' half-height VME boards. They used elevated sockets for the 68010 and placed DIP parts (bus buffers etc.) as well as decoupling caps underneath. Same thing for the peripheral/memory chips, they'd be raised and the PCB real estate underneath used.
Anything goes if you're space limited. Just do it.

djQUAN:
Might not be an issue but a possibility, the stress of inserting or removing a socketed chip could cause breakage of SMT components nearby and underneath especially long/big ones like the array or power inductor.
bson:

--- Quote from: T3sl4co1l on October 18, 2018, 12:47:47 pm ---I disagree, at least unless you're going to use 0201s.  0402s are the same size as a 1206 quad.  You do get more flexibility on where to place the individuals, if you need to fill in an oddly shaped space, of course.

--- End quote ---
Well, I'm looking at a row of 15 0402 resistors.  If I were to replace it with say a Vishay VSSR2403101JUF (QSOP-24, 0.64mm pitch) then it's narrower, about half the width, but the QSOP obviously has two rows of pads, plus 6mm of package or so on top of that.  Maybe I'm looking at the wrong parts?  A bunch of quads?  https://www.mouser.com/datasheet/2/427/vtsrvssr-88387.pdf

Here's what I'm looking at (ignore the silkscreen on vias):


Here's a QSOP-20 dropped on top for scale:


So, yes, it would fit in this particular case.  From a cost perspective, the 10x VSSR costs just over $1 qty 1k.  The Susumu RR resistors cost... $0.008 qty 10k (since it requires 10x as many).  Unless there's something to be saved during P&P I don't really see the economy here.  How can you compete with resistors that cost under a cent?  https://www.mouser.com/ProductDetail/Susumu/RR0510P-101-D?qs=sGAEpiMZZMvdGkrng054t%2fXDkuzyg5Mcu5v70Lq2dvM%3d  So $1000 vs $80? :-//

Edit: I do like the idea of being able to route on the top copper layer under the larger array package.  That can help in tight spaces as it effectively adds another layer.  I'll have to keep that in mind...  Of course the assignment to an array would be a choice made during layout, not during initial design (capture).
station240:
I wouldn't put a DC-DC converter under a CPU core, as it's just extra heat to get into the CPU.
technix:

--- Quote from: bson on October 23, 2018, 03:40:10 am ---
--- Quote from: T3sl4co1l on October 18, 2018, 12:47:47 pm ---I disagree, at least unless you're going to use 0201s.  0402s are the same size as a 1206 quad.  You do get more flexibility on where to place the individuals, if you need to fill in an oddly shaped space, of course.

--- End quote ---
Well, I'm looking at a row of 15 0402 resistors.  If I were to replace it with say a Vishay VSSR2403101JUF (QSOP-24, 0.64mm pitch) then it's narrower, about half the width, but the QSOP obviously has two rows of pads, plus 6mm of package or so on top of that.  Maybe I'm looking at the wrong parts?  A bunch of quads?  https://www.mouser.com/datasheet/2/427/vtsrvssr-88387.pdf

Here's what I'm looking at (ignore the silkscreen on vias):


Here's a QSOP-20 dropped on top for scale:


So, yes, it would fit in this particular case.  From a cost perspective, the 10x VSSR costs just over $1 qty 1k.  The Susumu RR resistors cost... $0.008 qty 10k (since it requires 10x as many).  Unless there's something to be saved during P&P I don't really see the economy here.  How can you compete with resistors that cost under a cent?  https://www.mouser.com/ProductDetail/Susumu/RR0510P-101-D?qs=sGAEpiMZZMvdGkrng054t%2fXDkuzyg5Mcu5v70Lq2dvM%3d  So $1000 vs $80? :-//

Edit: I do like the idea of being able to route on the top copper layer under the larger array package.  That can help in tight spaces as it effectively adds another layer.  I'll have to keep that in mind...  Of course the assignment to an array would be a choice made during layout, not during initial design (capture).

--- End quote ---
I am not using resistor packs for high precision purposes - just bus pull-up/pull-down, high speed signal termination and LED current limiting. No point getting that Vishal part, just those UniOhm ones at best.
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