Authored By:
C. Xu, R. Kopf, J. Smetana, D. Fleming,
Alcatel-Lucent, Murray Hill, NJ and Plano, TX, USA
Summary
As part of High Density Packaging Users Group (HDPUG) Pb-Free Board Materials Reliability Project 2, the moisture sensitivity of various lead-free laminates and the effect of moisture uptake on the material survivability through Pb-free reflow were studied using capacitance measurements and time-domain reflectometry (TDR) impedance measurements. WIC-20 coupons were used as test vehicles. In this paper, results from 20 different laminate materials will be summarized. The relationship between moisture uptake and material survivability through Pb-free reflow will be discussed.
Conclusions
Area averaged capacitance measurements can be used to detect delamination created in WIC-20 coupons under the aging conditions used in this work, while localized TDR impedance measurements do not provide sufficient sensitivity. Both moisture removal and delamination during thermal aging decrease the effective dielectric constant of the laminate and lead to an apparent capacitance decrease. Moisture ingress/egress occurs uniformly and can be described by the CCDM (capacitance change due to moisture) curve for a given board construction. Delamination, on the other hand, is generally not uniform across the various layers and typically occurs only in the inner layers.
The plot of capacitance change versus the layers and the use of CCDM curves provide an effective tool for separating the capacitance change due to delamination from the capacitance change due to moisture removal and can be effectively used for delamination detection and semi-quantification of the degree of delamination during assembly reflow.
Using the Lichtenecker Mixing rule, the detection limit of delamination using capacitance change has been estimated to be 0.2-0.6% void formation in the laminate under the experimental conditions used in this work. Delamination, eye brow or cigar voids, which generate less than 0.2% void between the Cu layers, can not be detected using capacitance change measurements. This detection limit can be improved by optimizing the test vehicles and improving the precision of the capacitance measurement, which will be one focus of future work.
The 27 samples showed significantly different moisture sensitivity and moisture uptake kinetics, which are determined by the interaction between laminate and water molecules. Contrary to conventional belief, no correlation between moisture uptake behavior and delamination propensity of the laminates is observed. The primary factor determining the thermal stability of multilayer boards during reflow is the type of laminate. The process condition of the multilayer board, which is not studied in this work, should also have a strong effect on the thermal stability of the board. WIC-20 coupons can be effectively used for monitoring the quality of the multilayer board production and will be further investigated in follow up work. The construction of the multilayer board determines the location of the highest thermal mechanical stress during reflow and therefore the location as well as degree of delamination.
Initially Published in the IPC Proceedings
|
