Analysis of Laminate Material Properties for Correlation to Pad Cratering

Pad cratering failure has emerged due to the transition from traditional SnPb to SnAgCu alloys in soldering of printed circuit assemblies. Pb-free-compatible laminate materials in the printed circuit board tend to fracture under ball grid array pads when subjected to high strain mechanical loads. In this study, two Pb-free-compatible laminates were tested, plus one dicycure non-Pb-free-compatible as control. One set of these samples were as-received and another was subjected to five reflows. It is assumed that mechanical properties of different materials have an influence on the susceptibility of laminates to fracture. However, the pad cratering phenomenon occurs at the layer of resin between the exterior copper and the first glass in the weave. Bulk mechanical properties have not been a good indicator of pad crater susceptibility. In this study, mechanical characterization of hardness and Young’s modulus was carried out in the critical area where pad cratering occurs using nano-indentation at the surface and in a cross-section. The measurements show higher modulus and hardness in the Pbfree-compatible laminates than in the dicy-cured laminate. Few changes are seen after reflow – which is known to have an effect -indicating that these properties do not provide a complete prediction. Measurements of the copper pad showed significant material property changes after reflow. Introduction There are different tests to evaluate the susceptibility of laminates to pad crater: hot pin pull, ball pull, ball shear, drop and bending. There are standards for these tests by industrial organizations; nevertheless pad cratering and pad crater susceptibility identification is still a concern for original electronic manufacturers. None of the published studies have characterized the laminate materials in the localized area where pad cratering occurs. Meilunas et al [1] tested different methods and laminate materials with low loss to evaluate the susceptibility to cratering. They worked with low, medium and high loss, and they showed that the material with low loss had crack initiation but it did not propagate, as was the case with the other laminate materials. They stated that it is hard to tell which test is best for evaluation of pad crater susceptibility. Wong and Silk [2] [3] used a variety of tests with different laminates, Table 1, that had different curing agents and filler under three different reflow conditions. The results of the tests showed limited agreement on rank order among the test methods, Table 2. A subset of these laminate samples is used in this study. From these earlier studies and experience, E and F samples have a higher susceptibility to pad cratering. Also, it has been consistently found that exposure to Pb-free reflows increases the susceptibility for all laminates. These expected results are compared to local material property measurements. The authors of this paper investigated whether there is a correlation between the mechanical properties of the laminate materials and the susceptibility for pad cratering. Table 1 Properties of Laminates Used to Study Susceptibility to Pad Cratering Laminate Resin Type Glass Transition Temp Tg (Centigrade) Decomposition Temp Td (C) Time to delam T260 (Min) Time to delam T288 (Min) Z axis Expansion (50 to 260C) % A Dicy Unfilled 170 DSC 300 10 2+ 4.4 B Phenolic Unfilled 175 DSC 330 >30 >5 3.5 C Phenolic Filled 180 DSC 335 >30 >15 2.8 D Phenolic Filled 180 DSC 340 60 10+ 2.8 E Phenolic Filled 210 DSC 365 30+ 10+ 3.4 F Phenolic Unfilled 190 DSC 370 60+ 10+ 3.6 Table 2 – Rank Order of Laminates in Various Tests Laminate Material Hot Pin Pull Test Cold Ball Pull Test Ball Shear Test Drop Test 4-Point Bend with AE A 1 1 1 1 1 B 2 3 2 1 1 C 3 4 2 2 1 D 4 2 2 3 1 E 5 2 2 2 1 F 5 5 3 2 5 Experiment Samples Two of the three samples tested in this study are Pb-free compatible, cured with a phenolic curing agent, Laminates E and F. Laminate A, a non-Pb-free-compatible dicy-cured laminate that was consistently superior in the rank order for pad crater susceptibility, was also included. To evaluate the laminate materials, samples were cut from the test vehicles that were designed and tested in a previous study [2]. Two locations were selected on the test vehicle; one location is referred to as surface and the other as cross-section. The surface sample was indented on a feature with exposed epoxy, the fiducial soldermask keepout area, as shown in Figure 1. The exposed resin has similar surface topology and chemical exposures (copper etchants, etc) as near a BGA pad. Figure 2 shows the area selected for cross-section measurement. This area includes BGA pads. Figure 1 Area Selected for Surface Measurement Figure 2 Area Selected for Cross-Section Measurement Figure 3 Cross-Section of Printed Circuit Board For the cross-section measurements, samples were cut, mounted in epoxy, ground and polished. Figure 3 shows the crosssection of the test vehicles, with different areas that correspond to the copper pad, epoxy and glass fiber bundle. Since it is very common to find the laminate crack initiation and propagation below the copper pad, this area was selected to take the mechanical properties measurement. In order to reduce the scattering of the mechanical properties introduced by the nanoindentation measurement, cross-section samples were polished until mirror-like. One rule of thumb in nano-indentation is the “Rule of 5%”, where surface roughness should be 5% of the minimum penetration of the indent, which for this study is a minimum depth of 500 nm. The surface of cross-section samples was flat in order to avoid any variability from the sample preparation and testing. In the case of the samples that were tested at the surface of the fiducial keep out, the surfaces are rougher. This surface was not metallographically prepared, has topology from the toothy copper that was etched away and was directly exposed to the copper etchant, soldering and cleaning processes. These did not meet the Rule of 5%. Test vehicles were separated in two groups: bare boards without reflow (as is) and after five exposures to reflow. Reflow samples follow a standard reflow production process at 260 oC ± 5 oC peak, for 10 seconds and TAL of 62 – 69 seconds. A 5 mil stencil was used to print SAC305 solder. Nano-indentation measurement A nano-indenter assembled with a continuous stiffness module (CSM) and a Berkovich tip was used for the mechanical characterization. After cross-sectioning, samples were indented on the copper pad and epoxy board material near the PCB surface between the outer copper layer and the first bundle of glass. Measurements of the mechanical properties -hardness and Young’s modulus -were acquired using the traditional Oliver and Pharr approach [4]