▁assemb lies . ▁ ▁The ▁co pper ▁post ▁stand - off ▁developed ▁for ▁the ▁C SP ▁market ▁has ▁found ▁a ▁home ▁in ▁high - dens ity ▁inter connect s ▁for ▁advanced ▁micro - process ors ▁and ▁is ▁used ▁today ▁by ▁IBM ▁for ▁its ▁CPU ▁pack aging . ▁ ▁Co pper ▁pill ar ▁sol der ▁b ump ing ▁Rec ent ▁tr ends ▁in ▁high - dens ity ▁inter connect s ▁have ▁led ▁to ▁the ▁use ▁of ▁co pper ▁pill ar ▁sol der ▁b umps ▁( CP B ) ▁for ▁CPU ▁and ▁GPU ▁pack aging . ▁ ▁CP B s ▁are ▁an ▁attract ive ▁replacement ▁for ▁traditional ▁sol der ▁b umps ▁because ▁they ▁provide ▁a ▁fixed ▁stand - off ▁independent ▁of ▁pitch . ▁This ▁is ▁extremely ▁important ▁as ▁most ▁of ▁the ▁high - end ▁products ▁are ▁under filled ▁and ▁a ▁smaller ▁st ando ff ▁may ▁create ▁difficulties ▁in ▁getting ▁the ▁under fill ▁ad hes ive ▁to ▁flow ▁under ▁the ▁die . ▁ ▁Figure ▁ 2 ▁shows ▁an ▁example ▁of ▁a ▁CP B ▁fabric ated ▁by ▁Intel ▁and ▁incorpor ated ▁into ▁their ▁Pres ler ▁line ▁of ▁micro process ors ▁among ▁others . ▁The ▁cross ▁section ▁shows ▁co pper ▁and ▁a ▁co pper ▁pill ar ▁( appro xim ately ▁ 6 0   um ▁high ) ▁elect r ically ▁connected ▁through ▁an ▁opening ▁( or ▁via ) ▁in ▁the ▁chip ▁pass iv ation ▁layer ▁at ▁the ▁top ▁of ▁the ▁picture . ▁ ▁At ▁the ▁bottom ▁is ▁another ▁co pper ▁trace ▁on ▁the ▁package ▁substr ate ▁with ▁sol