Effect of reflow profile and thermal cycle ageing on the intermetallic formation and growth in lead-free soldering

The formation and growth of intermetallic compound layer thickness is one of the important issues in search for reliable electronic and electrical connections. Intermetallic compounds (IMCs) are an essential part of solder joints. At low levels, they have a strengthening effect on the joint; but at higher levels, they tend to make solder joints more brittle. If the solder joint is subjected to long-standing exposure of high temperature, this could result in continuous growth of intermetallic compound layer. The brittle intermetallic compound layer formed in this way is very much prone to fracture and cold therefore lead to mechanical and electrical failure of the joint. Therefore, the primary aim of this study is to investigate the growth of intermetallic compound layer thickness subjected to five different reflow profiles. The study also looks at the effect of three different temperature cycles (with maximum cycle temperature of 25 0C, 40 0C and 60 0C) on intermetallic compound formation and their growth behaviour.. Two different Sn-Ag-Cu solder pastes (namely paste P1 and paste P2) which were different in flux medium, were used for the study. The result showed that the growth of intermetallic compound layer thickness was a function of ageing temperature. It was found that the rate of growth of intermetallic compound layer thickness of paste P1 was higher than paste P2 at the same temperature condition. This behaviour could be related to the differences in flux mediums of solder paste samples used.

Prehistoric Pinus woodland dynamics in an upland landscape in northern Scotland: the roles of climate change and human impact

Pollen, microscopic charcoal, palaeohydrological and dendrochronological analyses are applied to a radiocarbon and tephrochronologically dated mid Holocene (ca. 8500–3000 cal B.P.) peat sequence with abundant fossil Pinus (pine) wood. The Pinus populations on peat fluctuated considerably over the period in question. Colonisation by Pinus from ca. 7900–7600 cal B.P. appears to have had no specific environmental trigger; it was probably determined by the rate of migration from particular populations. The second phase, at ca. 5000–4400 cal B.P., was facilitated by anthropogenic interference that reduced competition from other trees. The pollen record shows two Pinus declines. The first at ca. 6200–5500 cal B.P. was caused by a series of rapid and frequent climatic shifts. The second, the so-called pine decline, was very gradual (ca. 4200–3300 cal B.P.) at Loch Farlary and may not have been related to climate change as is often supposed. Low intensity but sustained grazing pressures were more important. Throughout the mid Holocene, the frequency and intensity of burning in these open Pinus–Calluna woods were probably highly sensitive to hydrological (climatic) change. Axe marks on several trees are related to the mid to late Bronze Age, i.e., long after the trees had died.

Breast milk levels of zinc and omega-6 polyunsaturated fatty acids and growth of healthy Chinese infants

AIM: To examine the concentrations of zinc and omega-6 polyunsaturated fatty acids (omega-6 PUFAs) in breast milk, the impact of zinc on omega-6 PUFA metabolism, and the growth rate of infants. METHODS: Forty-one mother-term infant pairs from a rural area of northern Beijing, China, who were 1 month (n = 18, group I) and 3 months (n = 23, group II) old and exclusively breastfed, were studied. The dietary records and the concentrations of zinc and omega-6 PUFAs in the milk of lactating women and the increase in weight and length of their infants during 1 and 3 postnatal months were analysed. RESULTS: The dietary intakes of mothers in the two groups were the same, i.e. high in carbohydrate and low in fat, protein and energy. The maternal zinc intake was 7.5mg/d and thus reached only 34.6% of the current Recommended Nutrient Intake (RNI). The levels of zinc and arachidonic acid (AA, C20:4 omega-6) in the milk of group I were significantly higher than those in group II. Furthermore, significant positive correlations were found between the concentrations of zinc and AA in the breast milk and between the level of milk AA and weight gain. CONCLUSION: Zinc may be a co-factor and essential for essential fatty acids (EFA) metabolism. Thus suboptimal zinc intake may cause EFA imbalance. Further studies of Chinese rural mother-infant pairs are necessary to determine whether zinc supplementation should be recommended when lactation exceeds 3 months.

Peat, pine stumps and people: interactions behind climate, vegetation change and human activity in wetland archaeology at Loch Farlary, Northern Scotland

First paragraph: In 1993, a peat-cutter, Bruce Field, working on the blanket peat bank he rented from the Sutherland Estate by Loch Farlary, above Golspie in Sutherland (fig 1), reported to Scottish Natural Heritage and Historic Scotland several pieces of pine wood bearing axe marks. Their depth in the peat suggested the cut marks to be prehistoric. This paper summarizes the work undertaken to understand the age and archaeological significance of this find (see also Tipping et al 2001 in press). The pine trees were initially thought to be part of a population that flourished briefly across northern Scotland in the middle of the Holocene period from c 4800 cal BP (Huntley, Daniell & Allen 1997). The subsequent collapse across northernmost Scotland of this population, the pine decline, at around 4200-4000 cal BP is unexplained: climate change has been widely assumed (Dubois & Ferguson 1985; Bridge, Haggart & Lowe 1990; Gear & Huntley 1991) but anthropogenic activity has not been disproved (Birks 1975; Bennett 1995). It was hypothesized that the Farlary find would allow for the first time the direct link between human woodland clearance and the Early Bronze Age pine decline.