Gold Embrittlement Mitigation
There are both advantages and disadvantages to having a gold finish on your components. Gold does not tarnish, it is a solderable surface, has a long shelf life and does not oxidize. The disadvantage of gold finish on a component is that gold is porous, it dissolves in eutectic tin/lead solder and creates embrittled (weaker) solder joints. Gold embrittlement mitigation through lead tinning is the answer to this uncertainty as defined by the J-STD-001F criteria. See an article on this topic here.
So what is the problem? Gold embrittlement and voiding appears in SMT solder joints when using gold-finished components. This means that over time the components are not as reliable in terms of their mechanical bond strength to the PCB. Components at some point and under the right conditions, can fall of the PCB.
Gold embrittlement in solder joints shows up when a percentage ratio of the weight of the gold versus the weight of the solder and the going maximum number for the amount of gold has been 3% of the total weight of the solder alloy. (Recent findings indicate that gold embrittlement can occur at even lower percentages!)
In the J-STD-001 F version the criterion was changed in order to address the problems experienced by users. Due to the consistent lack of turbulence in the plated throughhole during the wave solder operation, these embrittlement problems can become more pronounced. Therefore there is a need for the gold embrittlement mitigation process.
Per the IPC-JSTD-001F, gold needs to be removed for BOTH Class 2 and Class 3 assemblies in the following cases:
- From all of the surfaces to be soldered of solder terminals plated with greater than 2.54 um in thickness of gold
- ALL solder cup terminals regardless of the thickness of the gold layer
- From 95% of all surfaces which need to be soldered regardless of how much gold thickness exists (new for version “F”)
- All throughhole leads which will be hand soldered (new for version “F”)
- From at least 95% of the surfaces to be soldered of THOLE component leads with 2.54um of gold thickness
When the components are manually soldered, the amount of solder being added to the solder joint in a supported hole is minimal due to the physical volume of the hole minus the volume of the lead could create a gold rich environment. This would leave it prone to crack propagation within the plated through hole.
Finally, if the components are surface mount components, the amount of solder paste added to the pads is small in volume. The ability of the gold to dissolve in that molten solder paste does increase the amount of gold per given volume of solder paste. This means that the Au dendrites within the solder joint would have a greater chance of serving as a catalyst for the propagation of cracks within the solder joint.
Therefore in order to be safe, removal of the gold eliminates the potential reliability impact concerns based for the exact amount of gold on the leads and terminals. Gold embrittlement mitigation can be done by BEST Inc. through lead tinning in order to ensure proper, long lasting solder joints can be formed.