Hand soldering, paste reflow, and wave soldering are the dominant forms of technology used to apply solder in the manufacturing of electronic assemblies. They pre-date the shift toward the greener, lead free processes now making their way into the mainstream of electronic manufacturing.

The transition process (Figure 6-1) presents a daunting number of possible permutations of board finishes, component finishes, and solder alloys that may manifest themselves in an assembly operation, most often at an inconvenient time. Engineers, operators, and technicians must be cognizant that there are different manufacturing parameters for producing lead free assemblies and the subsequent rework that follows.

Hand Soldering

One of the more common problems often overlooked with reworking a lead free solder joint, is the importance of pre-heating substrates prior to the application of the soldering iron to the joint area. The pre-heating will reduce the contact time of the soldering tip to the joint, which will mitigate the localized overheating of the underlying pad. A controlled temperature hot plate is a recommend tool for preparing the substrate for lead free soldering. The following conditions can be used for soldering SAC 305 (Sn 96.5%, Ag 3.0%, Cu 0.5%):

• preheat temperatures – 100 to 125° C
  
• soldering tip temperatures – 345° C
  
• tip dwell times less than 2 seconds

Another important aspect of securing a successfully re-worked solder joint, is to ensure total cleanliness of the area to be reworked. This includes using clean soldering tips, and removing old solder from both the component and pad area. Contamination of the newly applied solder will result in the formation of oxides that will prevent proper wetting and bonding. Often, to compensate for difficulty in applying a proper fillet, an operator may affix the soldering iron in the pad area for an excessive amount of time, causing delamination of the substrate. It is important to remember that the tip temperatures are considerably higher for lead-free soldering. Compensating with additional flux is not a panacea to correct improper technique and incorrect soldering parameters. Excessive flux can lead to cleaning difficulties and disproportionate formation of solder voids from volatile flux residuals.

Ball Grid Array (BGA) Rework

Among the commonplace areas where the transitional phase from Sn/Pb to lead free will impact production and quality, is in the placement of mixed solder BGAs. The EMPF has encountered numerous applications where a mismatch in the paste and BGA solder alloy, has caused the formation of cold solder joints. For example, the operator presets their profile to accommodate a eutectic paste alloy, while applying a SACB (SnAgCuBi) solder ball, resulting in a non collapsed BGA (Figure 6-2).

In situations such as this, it is recommended that the profile from the higher melt alloy be used to induce homogeneity in the formation of the solder. Temperatures are typically 30-40° C higher at the liquidus peak, than the eutectic Sn/Pb counterpart. Keep in mind, the desired peak temperature of the alloy can be substantially higher than its melting point, and will depend upon the efficiency of the reflow system to handle the gradient heat effect.

Some applications where a stenciled paste deposit is not warranted or prohibitive, the use of a tacky flux (which incorporates mixtures of flux and residual amounts of alloy), can be substituted. The profile, as exemplified in a typical BGA rework station, should have the appropriate zone parameters at the preheating, soak, and liquidus stages. As in any reflow process, it is imperative that a test board be sacrificed so that a proper profile can be developed. This is done by creating a “temperature envelope” around and directly under the BGA, by the strategic placement of thermocouples. This will ensure that temperature uniformity is being achieved across the zone of interest, and that no adjacent components are being adversely affected due to excessive heat.

The use of lead free solder and finishes has become essential in both new designs and field applications where reworked substrates and replacement components are required to keep pace with a changing COTS (commercial off-the-shelf) supply chain. The EMPF has been at the forefront of these changes, and is committed to helping our valued customers through the technical roadblocks.