What is Circuit Design

Circuit Designers try to make connections.

6. Putting it all Together

6.3. Intermetallic Bonds

The flux is solder-paste melts at a lower temperature than the metals.  Liquid flux flows over exposed base metals, removing the oxides and surface contaminants and providing a temporary barrier against oxidation.  Additionally, flux acts to lower the surface tension of liquid solder metal -- allowing it to spread further along flat surfaces and draw higher to form stronger fillets on vertical surfaces.  

Solder Reaction Layers

Flux helps to both protect the reaction layer from oxidation and to help extend its reach along a base metal.

When the solder-paste reaches a slightly higher temperature, solder-metals liquefy and begin to displace the flux as they move over the metallic surfaces.  While in this liquid form, the tin in the solder metals begins to form intermetallic bonds where it interfaces with copper pads, pins, and lands.  These intermetallics are what mechanically attach a pad/pin to solder, and solder to a land.

Intermetallics are crystalline arrangements of molecules that behave similar to pure metals.  Unlike an alloy, where elements are simply well mixed, intermetallics exist in exact stochiometric proportions.  For example: Cu6Sn5 and Cu3Sn.

Intermetallic Bonds

These example crystals show some of the different shapes molecules might arrange themselves in to form a crystal.

Intermetallics

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This scanning electron microscopy cross-section shows the mixture of tin and lead solder in an alloy and an intermetallic layer.

It's important not to allow the solder to remain liquid for too long, as large intermetallic crystals will form, and these crystals become points of weakness in a solder joint.

SolderJointFailure

This solder joint is in the process of failing due to excessive intermetallic formation, likely caused by prolonged melt-time during assembly.