The demand for Wafer Level Packages (WLP) has increased significantly due to its smaller package size and lower cost. However, board level reliability of WLP is still a major concern. This study investigates the board level temperature cycle reliability of three very different wafer level package configurations. Comprehensive studies are carried out through temperature cycle test, failure analysis, and finite element modeling. To assess the wafer level package capability and technology limit, the following parameters are considered: WLP structure, array size, ball locations, ball pitch, and temperature cycle profile.Daisy-chain chips are used for this study. Extensive failure analysis was carried out to confirm failure mechanism and quantify the mechanical degradation among different solder joints due to temperature cycle stressing. It is suggested that the primary failure mechanism is solder fatigue. The crack is in bulk solder at package side. The cracks initiate from both sides of the solder joint. The cracks propagate from edge toward the center of the solder ball. The corner balls are most susceptible to solder joint failures. Based on test data, making corner balls electrically not connected improves the WLP reliability by 30%.It is concluded that WLP structure A has superior solder joint reliability compared to B and C. For a given ball array size, smaller pitch gives better solder joint life. The solder joint fatigue life is approximately 1.2 times longer for 2 cycle per hour (cph) conditions than 1 cph temperature cycling condition.
ObjectivesTo identify the availability and variability of learning opportunities through patient presentations on an acute medical placement at a teaching hospital.DesignA prospective study evaluating all acute admissions to the Acute Medical Unit over 14 days (336 hours). Clinical presentations and the day and time of admission were recorded and compared with the learning outcomes specified in the medical school curriculum.SettingAn Acute Medical Unit at a London teaching hospital.Outcomes(1) Number of clinical presentations to the Acute Medical Unit over 14 days and (2) differences between the availability and variation of admissions and presentations between in-hours and out-of-hours.ResultsThere were 359 admissions, representing 1318 presentations. Of those presentations, 76.6% were admitted out-of-hours and 23.4% in-hours. Gastrointestinal bleeding, tachycardia, oedema and raised inflammatory markers were over three times more common per hour out-of-hours than in-hours. Hypoxia was only seen out-of-hours. Important clinical presentations in the curriculum such as chest pain and hemiparesis were not commonly seen.ConclusionsThere is greater availability of presentations seen out-of-hours and a changing landscape of presentations seen in-hours. The out-of-hours presentation profile may be due to expanded community and specialist services. Medical schools need to carefully consider the timing and location of their clinical placements to maximise undergraduate learning opportunities.
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