A Universal Input Using the THAT1570 and 5173

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: A Universal Input Using the THAT1570 and 5173

Post by mediatechnology »

Another potential issue with pads under ICs is any contamination or moisture that creates vapor pressure when heated could force solder out perhaps creating lead bridges. So larger holes under the part can somewhat mitigate any vapor pressure, while this is a pretty mature technology so I've never had the problems like some report.
On a couple of the early hand-built 1570 test demo boards we saw an increase in THD which was due to contaminates, that we thought was flux residue and/or moisture, trapped under the device. It was easily flushed out and the THD dropped. That may be why they added the fifth hole in the center of their pattern to serve as an additional weep hole.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: A Universal Input Using the THAT1570 and 5173

Post by mediatechnology »

promixe - Were you able to get any 5173 samples?
promixe
Posts: 27
Joined: Fri Apr 22, 2011 5:23 pm

Re: A Universal Input Using the THAT1570 and 5173

Post by promixe »

mediatechnology wrote:promixe - Were you able to get any 5173 samples?
Hi Wayne,

I haven't been able to get to ordering these yet, unfortunately. I'm finishing up some projects that are taking up most of my time right now, and will give Tom a call as soon as I'm done with them.

PS: Thanks for checking out Lucidity Preamp! Trying to document it now, which is not my favorite task to do - I'm much happier just building these. =))
juniorhifikit
Posts: 139
Joined: Sun Oct 04, 2009 8:37 am
Location: Oakland/Paris

Re: A Universal Input Using the THAT1570 and 5173

Post by juniorhifikit »

promixe wrote:Hi,

It's better to maximize the copper area under the chip to spread the heat better, therefore having the pad on the top and the bottom of the PCB joined by vias is a good start. My design has 4 vias to stitch the pads, not 5, the number doesn't matter that much at the end of the day. =) I have 4 to fall under the cheaper price bracket of my PCB fab house - that way I can have larger annular ring spec and looser tolerances, making production cheaper.
I did the same thing: 4 holes. The minimum drill size at the pcb fab' I'm using was .4mm, so I'd imagine .25mm from the data sheet would incur extra cost.
Also, and I wish someone told me this before, as I destroyed quite a few chips that way... =)) ...pay extra attention to the stencil apertures (solder paste layer). Make sure you don't extend the pad on the PCB to the pad boundaries of the chip (so, do NOT make the PCB pad the same area as the chip's pad! as it greatly increases possibility of solder shorts). Download Mentor Graphics' free LP Viewer and consult suggested pad dimensions. Also, contract stencil pad apertures by 1.5-2 mil and make the center pad window-style (i.e. 4 smaller squares, not a single blob). I've had good results with 5mil stencil height.
Great info! I reduced the size of the thermal pad's solder paste & solder mask a little bit to keep it away from the pads (red=top). I also added a bit or "restrict" to keep the ground plane's pour away from RG1 & RG2. I can't thank you enough for the tips! My only question now is, how big should the bottom layer pad be? (blue=bottom) I made it bigger than the top, but I'm not sure if that's a good thing... I also removed the thermals for the vias, but I'm not sure if that will make it too hard to solder. Any thoughts?

Image

Your preamp looks great! Seems like it could also work as an OEM type product in other company's designs.
promixe
Posts: 27
Joined: Fri Apr 22, 2011 5:23 pm

Re: A Universal Input Using the THAT1570 and 5173

Post by promixe »

Keep in mind that the pad is not ground (there is no ground pin on the chip), but Vee (V-), so I'm not sure if extending it that much will not affect stuff above it. Wayne might clear that up more.
Also, most PCB fabs have their basic annular ring spec at 12mil, which means that your via diameter has to be at least 40mil in size (not the drill size, but overall size). Visually your vias look smaller than 40mil, but I might be wrong. =)

Image

This is how I did it, blue is bottom, it just slightly extends over top but still doesn't overlap surrounding pads above it.

Image

Stencil apertures are light grey and copper is dark grey. I also shifted pin apertures away from the pad by 1.5mil (not shown here), as this is a standard industry practice for QFN.

Hope this helps. =)

PS: Yes, the preamp could totally be used in OEM, but its initial intended purpose is for DIY people to build a great sounding high-performance remote controlled preamp for under $100/channel.

A.
Post Reply