Assessing shelf life of components: Loss of solderability
Nearly all devices have electroplated Tin or Tin/Lead finishes that have not been reflowed. Assuming we do not have any plating problems as suggested by Gordon Davy solderability deteriorates by surface oxidation. Initially we only have oxidation of the Tin surface, and while this decreases the solderability, the surface will remain wettable to some degree. What really kills solderability is the intermetallic growing from the substrate through the coating and forming its oxide. Intermetallic formation is a reaction between the substrate alloy (copper, nickel or Alloy 42) and the tin in the coating to form an alloy. Intermetallic growth has two effects. Firstly, thinning of the remaining fusible coating: as the remaining fusible coating becomes thinner, surface roughness becomes an issue and wettability will decrease especially for reflow soldering. Secondly, when the intermetallic reaches the surface it forms an oxide which cannot be soldered by electronic fluxes. Hence the surface is unsolderable. With coatings in excess of 5 microns, intermetallic growth should not be a problem for at least 10 years.
Surface oxidation of the tin can be arrested and may even be stopped (moisture barrier bag and oxidation arrest paper), but intermetallic growth cannot be stopped, even in the freezer. (Putting components in cold storage, may cause condensation problems, so make sure the plastic bag is properly sealed).
Storage Life:
Intermetallic and oxidation growth can be predicted. However, the predictions are not always correct. This can probably be attributed to defects in the coating, which allow intermetallic growth to occur more quickly than predicted.
Therefore, I suggest a method using dry heat ageing and solderability testing using the wetting balance to determine shelf life. The suggested procedure is as follows:
Measure force @ 2secs from the non-wetting line (~-0.2mN, using the globule variant of the wetting balance) on receipt of the component.
Carry out ageing at 1550C depending on substrate, see table
Ageing time in days
Time 1 Time 2
Copper 4 25
Nickel 3 16
Alloy 42 7 60
Force @ 2sec for aged component should be > 40% of initial age
Time 1 > 5 years life
Time 2 > 10 years life
This criterion is good even for a storage environment of 400C & 100%Tin on Copper. The tin copper combination is the fastest one for growing intermetallic.
The above discussion mainly applies to storage. Assembled joints as such will not suffer solderability problems, although intermetallic growth will have some effect on thermal fatigue properties.
This knowledge has been gleaned from a long research programme at the National Physical Laboratory, UK, and can be found in books by Colin Lea and NPL reports.
Chris Hunt [log in to unmask]
National Physical Laboratory
Teddington, Middx, TW11 0LW, UK
Direct Tel +44 181 943 7027
Switch +44 181 977 3222 Fax +44 181 943 6453
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