SMTA International Conference Proceedings

Removal of High Density Plated Through Hole Connectors on Large Mass Multi-Layered Circuit Card Assemblies

Author: Michael Newman
Company: Harris Corporation
Date Published: 9/28/2014   Conference: SMTA International

Abstract: With the current technology thicker multi layered Printed Wire Boards (PWBs) backplanes that utilize many high-density Plated Through Hole (PTH) Connectors are becoming more common. Since most if not all United States aerospace Circuit Cards Assemblies (CCAs) are still using the tried and true Sn63Pb37 Tin Lead solder these backplanes are ending up with thicknesses over 0.120” and as much as 0.180”. To ensure a solid solder joint with no concerns over “Black Pad” these complex PWBs are manufactured with Hot Air Solder Leveling (HASL) finished PTHs. The cost of manufacturing these are expensive ($14+k) but more critical to any program is the lead time to replace the entire CCA. For any type of space related program these lead times could easily take months to re-order all components necessary to re-build the CCA. This forces the requirement of getting it right the first time.

Assembling these complex backplane CCAs is a task in itself that can only be done using automated Selective Solder Reflow Equipment. This equipment has the capability to pre-heat the CCA and keep it at a temperature that when the solder fountain does its work the amount of heat loss is minimized ensuring a through solder joint between the connector pins and PWB PTHs. What happens when a connector is defective or becomes damaged during assembly or any other reason that requires this connector to be replaced?

Traditional methods of using soldering iron and solder wick just don't work due to the thickness of the PWB and the typically large power and ground planes (2 – 3oz copper layers). Solder fountains might work if the PWB finish was Electroless Nickel Immersion Gold (ENIG) or maybe even Electroless Nickel / Electroless Palladium Immersion Gold (ENEPIG) but because of the HASL finish and the amount of time over the wave required to reflow the entire joint there is a real danger of dissolving the copper annular rings as well as the PTH barrels.

This paper will discuss the use of Hot Air rework equipment to effectively remove the solder and connector from these large mass high density backplanes without damaging adjacent connectors or any of the PWB metallization. Topics of discussion will include equipment selection, some general process parameters as well as some validation criteria and lessons learned.

Key Words: 

Rework, Backplane, PTH, HASL, Intermetallic

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