The best side to place decoupling capacitors tends to vary by package and PCB design. Some of these points have already been stated by our peers but I'd like to put this in more tangible terms by giving you examples.=20 The bottom line is that you generally want to provide the highest capacitance (or at least sufficient capacitance) with the least amount of series inductance. The capacitors themselves have series inductance as well so don't forget to consider this, but let's assume we are only using one capacitor. =20 In a conventional mostly digital layout, there will be many signals to route from the smallest package that can support these. This is usually laid out such that many signals are on the edge so that you can route many of them without vias in the PCB. This will typically force you to place any component side capacitors further away from a package since the area near the package will be dominated by routing. In this case, the capacitors are more effectively placed on the bottom side (opposite of the package) so that the series inductance to the package is minimized. Having many vias and capacitors will help in reducing the series inductance since you can divide it over more electrical paths. Therefore, in this case, it is best to have capacitors on the PCB backside below the package in the field of vias. A contrary case would be a typical high speed mostly analog board such as an optical network card with 5G, 10G or 40G signals. These types of devices will tend to have fewer signals and lots of shielding on the component side of the PCB. Here, your lowest inductance path for the capacitors may likely be on the top side since the wide shielding on the top layers will have less inductance than the path through the backside. Since VSS will normally be the shielding on the top layer, followed by VDD on the second or third layer (if VSS is used again on the second layer), the inductive path through the capacitors will just be comprised of very wide metal planes and a very short via (metal-1 to metal-2 or 3) twice. Not having to fit between the vias in the array below the package will also allow you to use larger less expensive capacitors. Therefore, in this case, having the caps on the top side would be better. You'll also need to consider caps to stabilize a PCB resonance or provide for local energy storage to prevent supply collapse if the power source is far from the component. For these, it doesn't really matter which side these types of caps are on. =20 Regards, Javier DeLaCruz =20 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Lee Ritchey Sent: Thursday, March 15, 2007 1:38 PM To: Jack Olson; sreekanthn@xxxxxxxxxxxxxxx Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: DecapPlacement If your planes are continuous, there is little benefit to placing capacitors on one side of a PCB vs. the other. All that might be achieved in a slightly lower mounting inductane based on how deep into the PCB the vias have to reach to connect to the planes. However, this delta L only moves the self resonant frequency of the capacitors a little higher or lower. In a normal PCB, this is not enough of a gain to warrant penalizing assembly by placing capacitors on the bottom the PCB, if so doing results if double sided assembly.=20 There are many papers published that support this. Lee Ritchey Speeding Edge > [Original Message] > From: Jack Olson <pcbjack@xxxxxxxxx> > To: <sreekanthn@xxxxxxxxxxxxxxx> > Cc: <si-list@xxxxxxxxxxxxx> > Date: 3/14/2007 8:07:08 PM > Subject: [SI-LIST] Re: DecapPlacement > > We put nearly ALL of our decoupling on the back. > Remember that even if the components are side by side, > the vias still have to get to the planes and back, making it > often the same electrical distance away. In other situations > the distance is even closer from the back because the > decoupling part can be placed directly under the power > pad, rather than being placed some distance away for > package size clearances. > best wishes, > Jack > > > On 3/12/07, Sreekanth N nampoothiri <sreekanthn@xxxxxxxxxxxxxxx> wrote: > > > > Dear Group, > > > > How much ll be the bad effect if we place a decap in > > the > > opposite side of the mother IC,if the design is in 200+ MHz range? > > Do we have any rule of thumb regarding the decap > > handling? > > > > > > Best Regards, > > Sreekanth N Nampoothiri > > > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > > > or to administer your membership from a web page, go to: > > //www.freelists.org/webpage/si-list > > > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > > > > List technical documents are available at: > > http://www.si-list.net > > > > List archives are viewable at: > > //www.freelists.org/archives/si-list > > or at our remote archives: > > http://groups.yahoo.com/group/si-list/messages > > Old (prior to June 6, 2001) list archives are viewable at: > > http://www.qsl.net/wb6tpu > > > > > > > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > List technical documents are available at: > http://www.si-list.net > > List archives are viewable at: =20 > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu