BGA Rework: Removal of Residual Solder
Manual Removal versus Semi & Fully Automatic Methods
After a BGA has been removed, residual Lead-Free solder remains behind on the pads which must be removed prior to the BGA replacement process. Commonly used manual solder removal processes involving soldering irons and desoldering braid (photo below) present significant issues when trying to remove higher melt point Lead-Free solder including:
- Lack of process control
- Damage to pads from metal-to-metal (wick to pad) contact
- Damage to solder mask
Damage to the solder mask is especially troublesome as it may be difficult for the operator to detect.
In addition to these issues, another significant issue associated with manual removal of Lead-Free solder is the growth of intermetallic phases. This issue was documented by the University of Rostock (Germany) in its study entitled “Comparative Research of Repair Soldering”.
The Time/Temperature chart (photo below), from the Rostock Study, depicts the removal of residual Lead-Free solder from the pads using desoldering braid. The area in red indicates the significant temperature variability associated with manual solder removal methods due to the lack of process control.
The Rostock Study compared the intermetallic phases on the pads after manual solder removal with braid to an alternate, semi-automated method. Magnified cross sections of the pads (photos below)
revealed that the thickness of the intermetallic phases after manual solder removal (3.7 µm) was more than triple, compared to an alternative semi-automated solder removal method (1.2 µm).
Excessive growth of intermetallic phases on the pads can impact the metallurgical composition of the solder joints when the replacement BGA is reflowed. In addition, the growth of intermetallic phases is inversely correlated to solder joint reliability. As a result, minimizing the growth of intermetallic phases should be a key objective of the entire BGA Rework Process.
The alternative solder removal system, used in the Rostock study, directs hot gas to the site to reflow the residual solder while an integrated vacuum tube removes the reflowed solder (photo below).
This alternative solder removal system provides the following significant advantages to manual solder removal methods:
• A vacuum sensor automatically and continuously adjusts the height of the vacuum tip, providing non-contact solder removal.
• The vacuum tip is a high temperature composite, eliminating metal abrasion.
• The nozzle temperature and flow is computer controlled.
• The solder removal pattern and speed is either fully automatic or manual, depending on the machine that is used.
Standard and micro size tools (photo below) provide the capability to remove solder from virtually any site.
Authors ~
Brian Czaplicki, Air-Vac Engineering Company, Inc., brian.czaplicki@air-vac-eng.com
Janos Tolnay, Zevac AG, j.tolnay@zevac.ch
References ~
Mathias Nowottnick, University of Rostock, “Comparative Research of Repair Soldering” (June 2007).