Green mould compounds: Impact on second level interconnect reliability

Abstract

The change-over from conventional to so-called green moulding compounds has gradually taken place in the last five years. These green mould compound materials have a lower thermal expansion and higher elastic modulus. As a consequence of the modified thermal and mechanical properties, an impact on the thermo-mechanical behaviour of the package-board assembly is expected (so-called Package Board Interaction: PBI). The CTE difference between the package and the board increases introducing the green mould compounds.This paper discusses the impact of the use of green moulding compounds to the second level solder joint fatigue cracking. The comparison with non-green mould compounds used in the past is done by experiments for TSOP's and by finite element modelling studies for a wide range of packages. The study reveals that the impact of the transition to green mould compound is substantial and can lead to early solder joint fatigue failures in applications.The paper also shows that tests on flexible PCB's underestimate the impact. In particular for assembly on rigid boards the mean life time may reduce to 20% of the life time obtained for non-green moulded packages IntroductionThe last decade new electronic materials are being introduced by component, PCB manufacturers and assembly plants to cope with the IC-technology evolution, the environmental legislation (RoHS) and the "going green" trend driven by market pressure. Two major changes associated with this migration are the elimination of lead and halogens in electronics.One of these trends has been the replacement of the mould compound of plastic packaged IC-components by so-called "Green Mould Compounds" (GMC) which are usually halogen-free, hence the name. The change-over has gradually taken place in the period between 2006 and 2010 and has reached a very high penetration level without being noticed by most of the component users. Although these new mould compounds have many environmental and technical benefits (less moisture absorption, improved die compatibility), they may endanger the reliability of electronics significantly.A high amount of research resources has been spent on the selection of new 'leadfree' solders and what their impact is on the long term reliability of the second level assemblies [1]. It was clear from the beginning that the change from SnPb solder to leadfree solder had a major impact on the reliability as the stiffness, yield strength, creep behaviour and resistance to fatigue fracture of these new materials are different.

Early fatigue failures in copper wire bonds inside packages with low CTE green mold compounds

“…A side effect of the lower CTE is that the E-modulus increases (due to higher content of silica particles). In reference [1], it was shown that the product of E-modulus and CTE is about a constant value. Fig.…”

“…As a low CTE overmold materials have some benefit regarding CPI, it is of interest to know from which CTE on . In reference [1], it was shown that the elastic modulus varies inversely proportional to the CTE of overmold. Therefore, six different overmolds are selected ( Table 4).…”

“…In an earlier publication [1], it was reported that introducing green mold compounds (GMC) with low coefficient of thermal expansion substantially reduces the life time of the second level solder joints. This is mainly due to the higher CTE mismatch between component and PCB by introducing low CTE overmold materials.…”

How the mold compound thermal expansion overrules the solder composition choice in board level reliability performance

“…As electronics can be subjected to temperature cycling, the larger expansion/shrinkage mismatch between the board and component will increase the loading on the solder joint providing the electrical but also mechanical connection between package and board. The concern of the low-CTE mold compound packages has been first reported in 2011 [1]. Only few other references show some results on different mold compounds [2][3].…”

Impact of laminate cracks under solder pads on the fatigue lives of ball grid array solder joints

“…Thus, for BGAs produced using these new low-CTE moulding compounds, also the part of the BGA outside the chip area will have a rather low CTE. Simulations done by imec have shown that this may decrease the fatigue life of the solder joints with up to 85% [5,6].…”