Reflow characteristics

Solder powders coated with a thin layer of Parylene exhibit a high degree of resistance to oxidation and to reaction with the flux contained in the solder paste without substantially interfering with the reflow characteristics of the solder. The powders are used as such or as solder paste. The preferred Parylene type is Parylene E, made from 4-ethyl[2.2]paracylophane since it melts below the melting point of the solder, i.e., about 180°C." The formulation of a typical solder pastes are shown in Table 2.4. 

There are three important Au-Sn intermetallics AuSn, AuSn2, and AuSri4, Eutectic points are at about 85 wt percent Sn (215°C) and 20 percent Sn (280°C). Gold can also form a ternary compound when joined with Sn-Pb solder, exhibiting a eutectic at about 175°C. This can radically alter reflow characteristics and solder-joint performance. Au, like Pb, is poorly soluble in Sn, which means that it dissolves in hot Sn, Pb, or Sn-Pb solder and precipitates out of solution during the liquid-to-solid phase change of the solder. When it does, it forms brittle, platelike crystals that appear needle-like in cross-section. " ... 

These polymers offer faster drying, superior colour and exterior durability when compared with alkyd coatings and are much less prone to cracking. The gloss achievable is somewhat less however, and reflow characteristics, when brush applied, are poorer leaving some brush marking. 

Retain adhesion while the circuit is flexed Wet and bond well to both the polymer film and the copper foil Have low moisture absorption Have good dielectric properties Have low or no-flow characteristics Be thermally resistant to solder-reflow temperatures... 

There are a few other materials that need to be covered for this discussion. The first is solder paste. This is a mixture of minute solder beads, flux, and other materials to give it specific rheological characteristics for dispensing and chemical agents for metal surface preparation. For surface-mount applications, it is typically stenciled onto PWB bonding pads, and then the electronic component is placed upon the solder paste deposit. The paste holds the component in place during the reflow process.The second is flux, which, as mentioned previously, is a key component of solder paste. The flux is a heat-activated chemical agent used to clean solder-able surfaces. Both paste and flux will be covered in subsequent sections of this chapter. 

Profile Dependencies. The solder paste composition, circuit board lay-up (number of layers, board material), component types, component count, and the nnmber of components per unit area on the board will dictate or influence reflow profile characteristics. Oven performance such as heating characteristics (number of heater zones and responsiveness of each module) will dominate the process and profile. Althongh the reqnirements of the in use determine each profile, how well the profile is maintained depends on the materials to be reflowed, the heating characteristics, repeatability health of process eqnipment, and the attention to detail by the process engineer. 

Other lead-free solder alloys which have been developed and evaluated by fee industry include Sn-Ag, Sn-Ag-Bi, Sn-Bi-In, and other ternary and quaternary alloys (Ref 24-33). The use of alloys containing indium (In) may be limited due to fee scarcity of indium and its high cost. The eutectic Sn-Ag alloy (melting temperature 221 °C, or 429 °F) was qualified for automotive applications in fee 1990s on ceramic thick film substrates and PWBs (Ref 3,12-15,34- 5). The microstructural characteristics of the Sn-Ag-Cu system under different reflow conditions and... 

Using Eq 7 and experimentally obtained K values from Ref 8 and 22, we may estimate the characteristic times for Cu dissolution into typical solder joints. Currently, the sizes of solder joints in flip chip, CSP, and EGA packages are approximately 0.1, 0.3, and 0.5 mm (0.003, 0.011, and 0.019 in.), respectively. If those solder joints are made of 63Sn-37Pb, and reflowed at 220 °C (428 °E), the characteristic times for Cu dissolution are 4.4, 2.6, and 0.9 min., respectively. If those solder joints are made of Sn-3.5Ag and reflowed at 250 °C (482 T). the characteristic times are 1.2, 0.7, and 0.2 min., respectively. As can be seen later, the interfacial IMC grows much faster after Cu saturates the solder joint. Before Cu saturates the solder joint, the IMC grows and dissolves simultaneously. Therefore, for a flip chip of Pb-free solder joints, the IMC will be exceptionally thick. 

Reflowed Conventional Leadless SMT Assemblies with Pb Contaminant. In Fig. 8, we have plotted cycles-to-1% failures for SAC paste assemblies with various Pb contaminant, versus cycles-to-1% failures for 100% lead-free SAC assemblies under thermal cycling conditions -40 to 125 °C (-40 to 257 °F), or -55 to 125 °C (-67 to 257 °F). When data points appear above the main diagonal, life for assemblies with Pb contamination is of a longer duration than that of 100% lead-free assemblies. The data (Ref 7, 8, 13) is for conventional leadless SMT components 20 Input/Output LCCCs and resistors ( R ) of sizes 0603, 1206, and 2512. The source of Pb contamination is the Sn-Pb HASL board finish, or the Sn-Pb component termination, in the 1206 resistors. In the 20 I/O LCCC SAC boards with Sn-Pb HASL finish, Woodrow (Ref 13) measured a 0.5% Pb contamination level. The cycles to 1% failure were calculated from Weibull parameters (characteristic life and slope of failure distributions) provided... 

In addition to wire harness connectors, printed circuit board-attached connectors, for automotive or electronic applications, also benefit from many of SPS s performance characteristics, including low viscosity, which helps to fill thin walls and heat resistance to maintain dimensional stability through lead-free reflow soldering operations (see Table 15.2). The trend toward miniaturization in automotive electronic control modules has increased the need for compact surface mount connector technology, as opposed to bulkier through-hole and wave-soldered alternatives. 

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