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7-9 October 2009, Leuven, Belgium Impact of Moisture Absorption on Warpage of Large BGA packages during a lead-free reflow process B. Vandevelde 1 , R. Deweerdt 2 , F. Duflos 2 , M. Gonzalez 1 , D. Vanderstraeten 3 , Eddy Blansaer 3 , Guy Brizar 3 , Renaud Gillon 3 1 IMEC, Kapeldreef 75, B-3001 Leuven, Belgium 2 Groep T, Leuven, Belgium 3 On Semi, Oudenaarde, Belgium Abstract- During a solder reflow process, IC components deform under a rather excessive temperature loading. A too high warpage at temperatures above solder melting, can cause either that the joints in the corner are not soldered to the PCB pads, or that the solder joints are shorted due to compressing of the corner joints. In this work, the warpage of a 35by35 mm 2 large PBGA package has been measured during a temperature profile which is similar to a lead-free soldering process. It was found that the warpage becomes very high when the applied temperature is above the glass transition temperature of the overmould material. At that moment, there exists a very large CTE mismatch between the overmould and the BT laminate. The warpage measurements have been successfully verified by Finite Element Modelling adapting the right material properties. This proves that modeling can be used as an estimator of warpage for packages. Also the impact of initial moisture uptake has been experimentally investigated and it was shown that it has a dominant effect on its warpage behaviour. Keywords – Warpage, PBGA, topography measurements thermo-mechanical stress, solder reflow, FEM simulation I. INTRODUCTION Packages consist of different materials, having a different coefficient of thermal expansion (CTE). Under a temperature change, the different mechanical expansions cause internally acting forces and this finally results in internal mechanical stresses and an out-of-plane deformation of the package. In this paper, the solder reflow process is considered as temperature profile, where components are subjected to rather excessive temperature loading. As a consequence, excessive warpage can cause shorts due to compression of neighbouring balls in the corner area under global warpage, as indicated in the schematic drawing in Fig. 1. For other package concepts, e.g. QFN’s, the opposite can happen that packages are warped upwards in the corner, resulting in open connections, or even worse, so-called frozen connections only made by mechanical contact, giving no failure at room temperature but starts to disconnect again at higher temperatures. 20°C 250°C (SAC reflow temperature) Fig. 1: Schematic drawing of shorts at the corner due to excessive warpage (happens during liquid soldering) This phenomenon of interconnection problems due to excessive warpage during solder reflow, is strengthen by the change to lead-free, which requires typically 30°C higher melting temperature, and also by moisture absorption, as will be shown in this paper. As warpage is typically proportional to the square of the package size, large packages are also more vulnerable. Also the overmould properties play a very important role: both its Tg (glass transition point) and the amount of moisture it will absorb, will determine the warpage profile.. II. EXPERIMENTAL WORK A. Sample description For this study, a PBGA package encapsulated by an overmould material has been selected. The size of the package is 35 by 35 mm 2 , the die size is 13 by 13 mm 2 . Fig. 2: Picture and schematic drawing of the PBGA 35x35 mm 2 package ©EDA Publishing/THERMINIC 2009 113 ISBN: 978-2-35500-010-2
7-9 October 2009, Leuven, Belgium Regarding the study of the impact of moisture absorption taken up by the package, the same package type has been Temperature (°C) measured with four different pre-conditions, as shown and 250 described in Table 1. For each condition, 5 samples have been measured. 200 TABLE I Overview of Sample Conditions for warpage measurements Condition Code Reference structure with uncontrolled “REF” history, as they were lying for more than 1 year lying on the shelf (probably, they took up some moisture) Reference structure dried for 3 days at “DRY” 120°C Reference structure humidified in a “WET1” humidity chamber, for 168h at 85°C and 85% humidity. Reference structure humidified in a pressure “WET2” cooker chamber, for 18h at 120°C and 100% humidity. B. Measurement setup The measurements were done with the Topography and Deformation Measurement (TDM) System from Insidix [4,5]. The out-of-plane measurements use the principle of the projection moiré technique to measure the topography as the difference between a reference horizontal surface and the measured surface. This reference surface is a calibration tool that is perfectly flat and homogeneous white and is measured beforehand. The outcome of one measurement is the out-ofplane profile, as shown in Fig. 3. The warpage is then defined as the difference in out-ofplane displacement between the center and the corner (not the difference in z-height!). 150 100 50 0 0 5 10 15 20 25 Time (min) Fig. 4: Temperature profile applied to PBGA sample for the topography measurement C. Measurement result for a “DRY” sample The first experiment presented in this paper is the warpage profile measurement for a dry sample. Fig. 6 shows the outof-plane profile for this package at 22°C and at 227°C (= maximum reflow temperature). At 22°C, the warpage is almost zero, while at 227°C, the warpage is visibly high. Z-height (µm) 1800 1600 1400 1200 1000 800 600 400 200 0 227°C 22°C 0 10 20 30 40 50 Distance from one corner (mm) Fig. 5: Out-of-plane profile (topography) measured by INSIDIX equipment (along the diagonal). Fig. 3: Topography measurement of the PBGA package B. Temperature loading profile A typical reflow temperature profile has been applied for this package, with a maximum of 227°C. The heating is performed by IR camera’s, while the temperature is measured using a thermo-couple touching the BGA sample. The warpage has been measured at different temperatures, both in heating and cooling. The result for this sample is shown in Fig. 6. At room temperature, there is a small warpage of about 50 µm (corners bending downwards). This warpage is almost constant up to 170°C. Above this temperature, the warpage substantially change and the corner start to bend downwards. This reason is that the glass transition for the overmould is reached, resulting in a high CTE mismatch between the overmould and the BT (while they are quite well matched below 170°C). As a consequence, the corners start to warp downwardsn and this becomes very high at maximum temperature (450µm). ©EDA Publishing/THERMINIC 2009 114 ISBN: 978-2-35500-010-2
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