Reflow Phenomena

When phosphorus is added to Si02, in addition to gettering mobile alkali ions, it tends to reduce the intrinsic tensile stress in such films, thereby reducing their tendency to crack. Both functions are important when the film is used as a final passivation film for integrated circuits encapsulated in plastic. Phosphorus additions of 7 weight percent seem to be optimum in order to produce the above desirable film characteristics. 

For interlayer dielectric, another consequence of the addition of phosphorus and/or boron is the ability to reflow the glass at lower temperatures. Lowering processing temperatures is a continuing objective in CVD processing. 

The top figures refer to the planarization of a deposit over a poly line. On the left is the as-deposited PSG. On the right, we see how much smoother 

Reflow experiments established that films prepared with 3.4 wt % B and 4.5 wt % P had the same flow behavior, at a given temperature, as a PSG film with about 7 wt % P. Therefore, one can either add more phosphorus to further lower the flow temperature or maintain it as is and have a more reliable process. 

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Koo, A., Komatsu, H., Tao, G., Inoue, M., Guth, P.H. and Kaplowitz, (1991). Contribution of no-reflow phenomenon to hepatic injury after ischaemia-reperfusion evidence for a role for superoxide anion. Hepatolt, 15, 507-514. 

Iwakura K, Ito H, Kawano S, et al. Chronic pre-treatment of statins is associated with the reduction of the no-reflow phenomenon in the patients with reperfused acute myocardial infarction, Eur Heart J 2006 27 534-539. 

T. Reffemann and R.A. Kloner, Microvascular alterations after temporary coronary artery occlusion the no-reflow phenomenon, Cardiovasc. Pharmacol. Ther. 9(3), 163-172 (2004). 

P. Golino, P.R. Maroko and T.E. Carew, The effect of acute hypercholesterolaemia on myocardial infarct size and the no-reflow phenomenon, during coronary occlusion-reperfusion, Circulation 75, 292-298 (1987). 

L. W. Klein, M. J. Kern, P. Berger, T. Sanborn, P. Block, J Babb, C. Tommaso, J. M. Hodgson, T. Feldman, Society of cardiac angiography and interventions suggested management of the no-reflow phenomenon in the cardiac catheterization laboratory, Catheter Cardiovasc Interv 60, 194—201 (2003). 

Ames A III, Wright RE, Kowada M, Thurston JM, Majno G. 1968. Cerebral ischemia. II. The no-reflow phenomenon. Am J Pathol 52 437-453. 

There are several potential mechanisms of PG involvement in the pathogenesis of acute renal failure (ARF). Vascular factors are pre-eminent in the initiation and maintenance of acute renal failure. These include control of RBF, autoregulation, tubuloglomerular feedback, the glomerular capillary ultrafiltration coefficient (Kf), the no-reflow phenomenon (which is secondary to capillary endothelial injury and swelling), the renin-angiotensin system and endogenous vasoconstrictors (e.g. catecholamines). 

Moisture is absorbed by adhesives and other plastic materials to various degrees and can accumulate in voids within the bond line during assembly, testing, and operation. The absorption of moisture in adhesives as well as in other polymeric materials, such as molding compounds, has been proven to cause failures when parts are subsequently solder reflowed and exposed to the high solder melt temperatures of 200 °C and above. The rapid evaporation and expulsion of the moisture result in stresses that cause cracking and delamination, a phenomenon referred to as popcorning. 

When massive, densely populated boards are introduced to the vapor reflow oven, vapor collapse can occur—a condensation rate that outpaces that of vaporization. The result is that the internal atmosphere of the oven thins dramatically to the point that it cannot sustain adequate reflow. Vapor-phase machines that rely on immersion heaters are prone to this phenomenon. More recent machines include massive heating element housings that provide sufficient thermal inertia to preclnde this problem. 

Ni-Cu-Sn Intermetallic Another phenomenon reported to cause a similar failure is based on the formation of a Ni-Cu-Sn ternary intermetallic in the solder joint. The Ni-Sn intermetallic forms on the component side during ball attach. When the part is mounted on the board, copper migrates from the board pad side, across the BGA ball, to form a Ni-Cu-Sn intermetallic on the component side. The thickness of this ternary intermetallic, typically 3 to 5 /tm thick, could grow with additional reflows. Clearly, an important factor contributing to this phenomenon is the surface finish on the PWB side. If the surface finish has a barrier like Ni to prevent copper from migrating across the BGA ball, this ternary intermetallic is less likely to form on the component side. 

Spalling. The IMC, initially formed at the substrate/liquid solder interface, sometimes starts to move away from the interface as a result of prolonged or multiple reflows. This phenomenon often seems related to Ni, especially electroless Ni(P) substrate. 

In addition to termination-material or compatibility issues, there are several other component material-related concerns, the most important of which is damage due to moisture uptake when processed at the higher Pb-free assembly temperatures. The most prominent defect is delamination due to warpage or popcoming, a phenomenon whereby the moisture absorbed by a component on the manufacturing floor is converted to steam when exposed to the solder reflow-temperature cycle. Many components are downgraded in moisture-sensitivity-level rating when used with lead-free process conditions. 

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