Soldering Rework

Isotropically conductive adhesive interconnections can be repaired using techniques similar to those used for solder rework. By application of heat locally at a temperature above the T value, a section of adhesive can be softened and the device can be removed... 

Decreasing or eliminating thermal shocks by preheating the board before hot-air leveling, wave soldering, rework with a solder pot, etc. 

Managing the compatibility issues is critical to lead-free transition. These include materials compatibility (solder, components and PWB), process compatibility (reflow, wave soldering, rework, equipment, and yield), design compatibility, reliability compatibility, and business compatibility (cost, supply chain, and operations). 

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. 

The NEMI Task Force is examining issues of process control, component survivability, wave soldering, rework, supply chain issues, and reliability. The NEMI Task Force project includes thermomechanical fatigue testing from —40 to 125°C for six component types, and from 0 to 100°C for a more limited component set. 

Engineering rework is possible with eutectic and solder materials, but impossible with silver—glass. This constraint severely limits the usefulness of the material. Tables 4 and 5 give the electrical, mechanical, and thermal properties for various adhesives. 

Ease of reworking by melting with a hot solder iron. 

Reworking underfilled flip-chip devices or BGA parts also presents new challenges since both solder and adhesive are present and both must be removed. New underfill formulations have had to be developed that soften, but do not char at the temperatures required to liquefy the solder. Some equipment manufacturers have modified their flip-chip bonders to include a rework station, In general, the chip undergoing rework is spot heated to... 

Chemically inert to flux, solder, and cleaning materials used in the process Curable as appropriate to the process UV, oven, or air cure Removable for rework and repair... 

In selecting suitable PCB material it is necessary to bear in mind that it will usually have to withstand at least three exposures to high temperature during the solder reflow process. One to each side of the board and a third after post test rework. Another potential problem with substandard laminated boards is that the laminations may separate when exposed to high temperatures. 

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