Quality-Technology-Series (*
1
):
Trouble prevention measures for ion migration during solder mounting
(*
1
) This series explains the specific examples (design, process, construction method, handling, etc.) in which the product fails to
maintain the function and the entire set becomes defective, as well as the reason why the product becomes defective.
1. What is ion migration?
When voltages are applied to electronic components such as a crystal unit under a high moisture
conditions on PCBs, metals that are ionized between electrodes are moved, which causes short circuits,
and ion migration (*
2
) occurs.
In recent years, electronic components are becoming narrower in pitches and fine patterning of
wiring. As a result, the size of crystal unit also decreases and the distance between external terminals
becomes narrower. Depending on the solder mounting conditions at the customer's site, ion migration
may cause short-circuiting between terminals.
(*
2
) ion migration is also known as electrochemical migration.
2. Generation mechanism and generation factors of ion migration
Metal contained in electronic materials such as solder receives electrons at the anode side when a
voltage is applied between electrodes of the metal in an environment where moisture is present, and
metal ions are dissolved from electrodes surface, and as it precipitates again as a metal, a phenomenon
that grows toward the cathode can be seen. When this precipitate is observed by SEM, etc., it appears
dendritic and is called dendrite. The susceptibility to the generation of dendrite differs depending on
the metal, and Sn, Pb, Ag, and other solder components are all said to be easily grown dendrite metals.
When a polar substance such as water or organic solvent exists between electrodes, the metal ionizes
and dendrites grow. Accordingly, reliability tests like high-temperature , high-humidity tests
accelerate the growth of dendrites, and in a short period of time, the dendrites cause insulation failures
due to short circuits between electrodes. When the flux residue during soldering is between electrodes,
moisture adheres to the cracks of the dry flux residue, which promotes the growth of dendrite.
<Ion migration generation conditions>
(1) Potential difference between electrodes
(2) Ionizing metals moving between electrodes
(easily generated; not generated for Ag>Pb>Cu>Sn>Au Fe, Pd, Pt)
(3) Humidity (moisture)
<Conditions for accelerating ion migration>
(1) High temperature
(2) Residue, contamination, dust, Br, active impurities such as Na
+
,Cl
-
,NH
4
+
, etc.
(3) PCB material
The reaction mechanism of ion migration is explained in the case of Ag as shown in the following
three steps.
Step 1: Ag elutes at the anode and electrolysis of water occurs at the cathode.
Anode: Ag → Ag
+
+ e
-
Cathode: 2H
2
O + 2e
-
→ H
2
↑(gas) + 2OH
-
Step 2: Ag
+
ions eluted at the anode and OH
-
ions generated at the cathode migrate to the
cathode while maintaining the equilibrium reactions of the following equations.
AgOH decomposes into Ag
2
O and disperses colloidally.
2Ag
+
+ 2OH
⇄
2AgOH
⇄
Ag
2
O + H
2
O
Step 3: Ag
+
ions transferred to the cathode receive electrons and precipitate in a dendritic
form.
Anode: Ag → Ag
+
+ e
-
Cathode: Ag
+
+ e
-
→ Ag ↓ (precipitation)
3. Ion migration example of crystal unit
If a high-temperature and high-humidity bias test (85°C 85% 1,000H) is conducted with flux
residue after soldering (lead soldering) of small crystal unit onto PCBs, migration occurs because the
conditions for generation and acceleration of ion migration are satisfied.