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.

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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 migra tion.

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-,NH4+, 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 : 2H2O + 2e-  → H2↑ (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 Ag2O and disperses colloidally.
            2Ag+ +2OH ⇄  2AgOH ⇄  Ag2O + H2O

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)

The reaction mechanism of ion migration

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.
  Fig. 2 and Fig. 3 show the external view of NX2520SD (external dimensions: 2.5 mm x 2.0 mm) samples used for high-temperature and high-humidity bias test. Dendrite is formed and the Hot and GND terminals are short-circuited. Dendrite in this case was found to be Pb, which is a solder component.

Fig. 2 Mounted with flux residue

Fig. 2 Mounted with flux residue

Fig. 3 Dendrite-formed crystal unit

Fig. 3 Dendrite-formed crystal unit

4. How to measure ion migration of crystal unit
  When measuring the continuity resistance to verify ion migration, using an insulation inspection meter or similar equipment may cause a high current to flow between the measurement area , resulting in burnout of the migration section, which may not reproduce the problem. It is recommended to measure with a low current of 1μW or less using a network analyzer, etc.

5. Preventive measures against ion migration problems
  Though the non-washing after PCB soldering is becoming popular, consideration should be given to prevent the generation of ion migration. Even when using a non-washing type solder paste, it is recommended to perform reliability confirmation such as high-temperature and high-humidity tests beforehand, because some conditions such as temperature and humidity may cause problems due to ion migration. Also, to prevent ion migration, consider the following:

(1) If the product is used under high temperature, high humidity, and energized conditions, wash the
     PCB and remove the flux.
(2) After cleaning, coat with moisture-resistant resin.
(3) When used without cleaning, a type of flux with low corrosivity and reduced active force after
     soldering should be used.

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