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Understanding and Using the PDR Rework system
This section has 4 main parts
Reference Information – The Science in SMT Rework
Principles of Operation
Control and Settings
Operating Procedures – Preparation, Soldering and Desoldering
Reference Information
There are Four sections here - The Science in SMT Rework, General Principles, Control and Settings, Alignment
The Science in SMT Rework
A lot of time, money and intellect is employed in developing a successful SMT production process. More needs to be
directed towards rework. Why, because post production soldering/rework is a fact of life for many reasons. The cause
of 80% of field failures is down to 'reworked' joints and you either scrap a lot more product, or sort out the
'soldering/rework' process by introducing industry standard, best practices. This article explains some of the science
involved.
The metallurgical bonding process of Soldering
Soldering is a metallurgical bonding process where two metal surfaces (e.g. copper pcb pads, and copper component
leads) are joined together by bonds formed with a 'bonding/filler' material (a suitable metal alloy i.e. tin/lead or a
lead-free solder), which is heated above its melting point and below the melting points of the metals being joined.
The bonds are formed by the formation of intermetallic compounds (an irreversible chemical process), and/or by
diffusion or absorption (a physical process).
When joining eutectic tin/lead solder (63Sn/37Pb) and other high tin alloys (including lead-free alloys) with copper,
various intermetallic compounds are formed - on the copper side and on the solder side.
The intermetallic compounds of copper and tin form crystalline grains (in layers), whose structures are determined by
the length and intensity of the thermal interaction. Short reaction times form fine grains, which promote good
solderability and solder joint strength. Long reaction times can result in coarse grains, and a thick intermetallic layer.
A thick intermetallic layer creates poor solderability and joint strength, affecting the mechanical strength and long-
term reliability.
Although the integrity of a solder joint is normally considered to be dependent on the thermal process during
assembly, metallurgical reactions can also take place during storage. Intermetallic layers continue to grow
coarser/thicker even at ambient temperatures. Therefore, when parts or boards are solder coated or pre-tinned,
prolonged or improper storage cause these layers to grow, severely affecting the solderability. Alternative lead
finishes and passivated copper pads can be used to combat these problems.
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