Effect of organic package warpage and assembly challenges using thin core substrate

Abstract

The need for thin core substrates is increasing in the industry to meet low inductance. However, there are major challenges of reducing thin core substrate warpage in assembly which needs thorough understanding of the assembly process steps and its effect on warpage. There are various assembly stages that influence the package warpage as a result of time and temperature. Reflow process during die attach, underfill curing and lid attach process play a significant role in package warpage. Substrate manufacturers typically provide substrate warpage of 4 mils max which does not allow much margin left in assembly considering the number of reflows and curing profiles the package undergoes during assembly. This paper illustrates the warpage at each assembly process step and its impact on assembly yield and reliability with 400 um thin core 27 x 27 mm, 4-2-4 package. In this paper, package warpage at various reflow profiles, underfill curing profiles and lid attach processes has been evaluated. The warpage studies are carried out on a flip chip organic BGA package with underfill and a single piece lid using a 400 um thin core substrate. The package also comprises of low k layers which is another challenge that is explored to ensure the reliability of low k delaminations on a thin core substrate. The challenges are increased further since the package also needs to qualify board level reliability in addition to level 1 qualification. The paper also illustrates the board level reliability tests and its qualification results.