Time above liquidus

The process window for lead-free soldering requires higher peak reflow temperatures and longer times above liquidus. Soldering in a nitrogen environment improves wetting and reduces oxidation of the flux residues and the solder. Due to the higher reflow temperature. 

It is important to limit exposure to high temperatures and particularly time above liquidus, as it is in this regime that the intermetallic layer growth is most vigorous.The thicker the inter-metallic layer, the more brittle and less reliable the solder joint wiU be. 

Reflow and Removal. The same considerations apply for this as for any reflow step. The ramp, soak, peak temperature and time above Uquidus are dictated by the solder alloy used and should be roughly the same profile parameters that were used for the initial assembly of the board. Once the component has reached complete reflow, the vacuum pickup tool is engaged and the component is lifted vertically. Care should be taken to minimize the time above liquidus to minimize intermetaUic compound thickness. 

There is very little information in the literature on the process consistency of backward-compatible assemblies and their consequent long-term reliability. Some information can be found in References 28,31,32 and 34. Some reliability data have been shown to indicate that the reliability of backward-compatible assemblies can be significantly improved if the Sn AgCu ball melts fully and good dissolution with the Pb/Sn paste is achieved. However, the factors that influence the degree of dissolution, such as the peak temperature and time above liquidus (TAL), are generally difficult to control consistently. Moreover, the amount of dissolution obtained is a function of the solder ball volume relative to the volume of paste used. Consequently, packages with smaller solder ball pitch could also be more sensitive to assembly process variations in backward-compatible assemblies. 

ICA isotropic conductive adhesive TAL time above liquidus... 

Overall rework time was about eight minutes for lead-free and six minutes for SnPb profiles. In most cases, board temperature 150 mils from the reworked component was above the liquidus-reflow temperatures, which was also the case during SnPb rework. Time above liquidus (TAL), which was often close to 90 seconds, combined with higher peak temperatures for lead-free solder rework, let to increased solder joint voiding. More solder paste development work is needed to support the elevated lead-free solder-temperature profiles. 

TC Location Time Above Liquidus (sec) Peak Temperature (C) ... 

SAC alloys offer solder joints that are less reflective than 63/37 the contact angles tend to also be higher and spread less. These are not considered defects, but cosmetic attributes. If air reflow is used, SAC joints will be less bright and will show surface effects such as crazing. These are due to the intermetallics within the solder and oxidation effects. If nitrogen reflow is used, the joints will be more reflective with enhanced spread. Lower peak temperatures and lower times above liquidus reduce intermetallic growth, but increase the overall solder-joint brightness. 

Reflow time, sec - 40 sec. dwell (time above liquidus temperature)... 

In general, the thermal profile created for the reflow furnace contains several distinct regions namely the initial ramp, a dwell at elevated temperature, a ramp to the maximum temperature, and a cool down region, as illustrated in Figure 39. The critical reflow profile parameters that must be controlled are the peak reflow temperature, oxygen level, dwell time above liquidus, soak time, ramp rate, cooling rate, conveyor speed, and the temperature difference across an assembly (AT). If the ramp rate is too low, the assembly may not attain the required soak temperature soon... 

Par] DTA with cooling and heating rates of 0.5 to 3°C min EMPA, light microscopy. The mixed powder were pressed and then held in a graphite crucibles at different temperatures above liquidus for different times. Fe rich comer at 0 to 24 mass% Cu and 0 to 5 mass% C. Isothermal section at 950,1000, 1150, 1155, 1172, 1200, 1400, 1450°C. Temperatures and constitutions of two invariant equilibria. 

FIGURE 48.2 Soldering steps required for first-pass soldering of a bottomside component on a PWB and success reflow steps encountered during first-pass soldering and repair. Note that the intermetallic compound formation thickness is not truly linear with each step. Thickness depends on materials of the soldering system, time above solder alloy liquidus, peak temperature, etc. The illustration is meant to show that with each reflow cycle, there is increased intermetallic layer thickness. 

To aid the solder fountain process, dense assemblies can be batch-heated in a box oven to make the repair process more efficient and limit exposure of the board to the turbulence of the molten solder wave. Limiting the soldered assembly s time near or above solder Uquidus is key for solder joint and product rehabUity. As mentioned previously, time above solder liquidus along with peak temperature will define the intermetallic thickness for a given metals system, and intermetaUic thickness will in part define the solder joint reUabihty.The turbulence of the... 

The composition of the electrolyte is shown in Table 1.8.1. Two hundred grams of electrolyte were added into the quartz crucible. Temperature was controlled at 955 1 °C during the experiment, giving the electrolyte an initial superheat above liquidus of about 3 °C. Two grams of alumina sample was charged into the transparent molten salt at one time. When the previous alumina had been completely dissolved and the temperature returned to the constant 955 °C, another addition was done until the newly added sample dissolved slowly or crucible was seriously damaged by bath corrosion. 

Only 45 s above liquidus (minimum allowed reflow time) was used for each reflow cycle, in order to limit the amount of time the board was subjected to the elevated temperatures. 

Takemoto, T. Miyazaki, M. Effect of excess temperature above liquidus of lead-free solders on wetting time in a wetting balance test. Mater. Trans. JIM 2001, 42 (5), 745-750. 

In one study [29] the formation of intermediate alloys in Sn-3.4Ag-0.7Cu/metal systems was investigated for Ni and Cu metallization (with the same die and Ni-V die metallization). In producing these samples, a Cu-Ag-Sn solder paste was printed onto Ni-V die terminal pads and reflowed. The resulting solder bumps were fluxed and reflowed a second time in order to attach them to an OSP-coated Cu metallization or Ni/Au/Cu metallization on a chip carrier. The chips were reflowed under standard industrial conditions. A second reflow was conducted to attach the solder to either a Ni or a Cu metallization. This reflow consisted of peak temperatures between 234° and 250°C, and times above the liquidus temperature ranging between 50 and 80 sec. 

FIG. 33 Comparison of the growth of intennetallic alloys for different reflow times with Sn-Ag-Cu solder. The Sn-Ag Cu solder was reflowed on chips with NiV under bump metallization. Then the bumped chips were reflowed to substrates with Cu/OSP pads above the liquidus of (a) 545 , and (b) 745 °, and a peak temperature of 238 C. An increase in time above the liquidus resulted in an increased intermetallic compound formation. 

TABLE 36 Peak Temperatures and Times Above the Solder Liquidus Temperature Used in a Systematie Reflow Process Parameter Study with Sn-3.8Ag. 7Cu... 

In addition to the metallurgical aspects discussed above, process parameters can have important effects on reliability as well. Good-quality paste printing, peak reflow temperature, dwell time above the liquidus temperature, cooling rate, and solder atmosphere reactions all affect the microstructure of solder joints and ultimately their reliability. Higher processing temperatures increase the dissolution rates of metal finishes and/or conductor metals (i.e., minor elements) into the solder and can increase the rate of intermetallic compound formation in the bulk solder. This increases the joint stiffness (i.e., reduces the compliance). 

Deposition Procedures. A schematic of a typical growth sequence is shown in Figure 6. After baking the system (step a), the system temperature is adjusted for growth and the substrate is brought into contact with the melt in the first bin (step b). In normal LPE practice, supersaturation of the liquid solution is achieved by temperature adjustment. The initial temperature can be set above, at, or below the liquidus value. If meltback is desired, a small increase in temperature above saturation is fixed first. The subsequent rate of etching decreases with time as the composition locally adjusts towards the equilibrium value. 

The composite, metal substrate, and braze foils (two layers, -100 pm total thickness) were sliced into 2.54 cm x 1.25 cm x 0.25 cm pieces and ultrasonically cleaned in acetone for 15 min. The braze foil was sandwiched between metal and composite, and a normal pressure of 1.2-4.7 kPa (0.38-1.5 N) was applied to the assembly. The assembly was heated in a furnace to -15-20 °C above the braze liquidus under vacuum (10 torr), soaked for 5 min., and slowly cooled to room temperature. The brazed joints were prepared for metallography and examined with a Scanning Electron Microscope (SEM) coupled with energy dispersive x-ray spectroscope (EDS) on a JEOL-840 A unit. Microhardness scans were made with a Knoop micro-indenter on a Struers Duramin A-300 machine under a load of 200 g and loading time of 10 s. 

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