Solder reflow

Protective tapes used during solder reflow proeessing. 

Figure 12.4. Left—Scanning electron microscope image of the input/output pads of a silicon IC bumped with eutectic lead/tin solder after solder reflow. Right—Photograph of printed wiring board interconnect pads that have been printed with lead/tin solder prior to reflow.

Figure 1. Intermetallics after solder reflow— elemental analysis by electron probe.

Figure 2. Different Cu-Sn intermetallics formed on solder reflow-X-ray diffraction.

Figure 3. CujSn intermetallic stress vs. number of solder reflows-X-ray diffraction.

Because flux is such a concern, one contractor is exploring ultrasonic bonding (10) using prepunched aluminum interconnects that are attached to electroplated copper cell metallization with a seam welder. Others are examining fluxless bonding concepts, such as vapor-phase solder reflow. 

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... 

High-temperature exposure during solder reflow (200-240 °C)... 

A fourth generation of electrically conductive adhesives based on cyanate esters and modified cyclo-olefin thermoset (MCOT) resins was introduced in the 1990s. These resins were specially formulated for low moisture absorption, high thermal stability, and low stress to be compatible with solder-reflow conditions. They were developed to prevent or reduce the so-called popcorn effect, a failure mode attributed... 

Cyanate ester and MCOT adhesives introduced for low moisture absorption, compatihle with solder-reflow temperatures (popcorn resistant)... 

In spite of their low polarities, cyanate esters adhere well to metal conductors and maintain good adhesion up to 250 °C. They are thus compatible with solder-reflow temperatures. The adhesion strengths of toughened cyanate-ester adhesives, as measured from aluminum-to-aluminum lap-shear specimens are reported to be 5,900 psi at 25 °C, 2,572 psi at 67 °C, and 4,000 psi at 82 °C. 

Aqueous and semi-aqueous cleaners are used primarily to clean printed-circuit boards prior to assembly, during application of the solder paste and adhesive (if they have been misprinted or smudged), and after assembly and solder reflowing. Highly automated multi-stage cleaning consoles are used in production where the parts are cleaned in a series of baths ending up with one or more deionized water rinses. 

Short cure cycles and ability to be cured in-line or simultaneously with solder reflow... 

After the solder has been reflowed and the adhesive has been cured, the adhesive has served its primary function of holding the components in position during solder reflow. However, since the adhesive remains permanently on the board, its cured properties must be such that it will not reduce the reliability of the assembly or cause subsequent failures. 

Area array on PWBs using solder-reflow cycle for underfill curing... 

A third approach to underfilling involves applying an insulative thermoplastic preform (film) prior to attaching the device. Under pressure and heat, the preform softens and flows around the solder bumps, then solidifies quickly on cooUng. The devices may then be solder reflowed to make the connections or the preform may be formulated to flow and encapsulate the solder during solder reflow. Table 5.6 lists some commercially available preforms used as underfills. Anisotropic film adhesives have also been used as underfill preforms, in which case z-direction conductive paths are formed beneath the solder bumps at the same time that the rest of the insulating film flows and cures around the bumps. Anisotropic paste adhesives can be similarly used (Figure 5.11). 

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. 

Moisture absorption in uncured and cured adhesives is important for reliable adhesive bonding. Absorbed moisture in adhesives, particularly those used in surface-mounted parts, may result in voids in the bond hne due to moisture desorption or entrapment. Absorption of moisture after cure also affects reliability. Popcoming, the cracking of plastic parts during solder reflow, is due to the rapid vaporization and expulsion of moisture retained by the die-attach adhesive or the plastic encapsulant. 

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