homeA failure occurs, using an electronic component for a long time on field use. One of thess failure includes an insulation failure with low electrical field. It was found that the insulation failure occured with a cause for the metal element of the electrodes moves on the insulotor. These type of failure causes a short circuit eventually.
Such a phenomenon was first reported in detail at 1955 by Kohman. The failure caused by this phenomenon was observed in a component for a connection of telephones, namely the silver atom or ion was moved on an insulator between silver electrodes and insulation resistance decreased. On the varing experiment works in this field, it was concluded that this silver transports, named "silver migration", occurs by an electro chemical mechanism. As the result, migration occurs in a humid environment, and under the infuluence in applied electric field. Silver is a very sutable material for electric curcuit and is used for many products. However, silver was known to undergo quick migration leading to insulation failure, and this is also the case of copper and other metals. In order to prevent this migration, researches were performed to elucidate the mechanism, which protect method under the infuluence and finding an sutable material.
Today, for electronic component, these metals are used inevitably. Therefore, it becomes an important problem to prevent a failure occurrence caused by this migration. In the following, a breaf summary of experimental result is given, as well as to be resolved.
1. Two types of migration
Migration of an electronic component has two types on the different condition of occuring. One is named "ionicmigration", and another is named "electromigration". Special feature of both are discribe as follows.
a. ionicmigration
・An ambient temperature is low ( less than 100 ℃ ).
・Occurence with a small current ( less than 1 mA/cm2 ).
・Necessity for moisture.
・Seeing of a partial metal of silver and copper and so on.
b. electromigration
・An ambient temperature is expensive ( more than 150 ℃ ).
・Occurence with a large current ( more than 104A/cm2 ).
・Seeing of many metals.
Thus, there is a difference for two migrations. In actual field, former maigration is become a problem. So, this report is described about an ionicmigration. For a simplification, this phenomenon is described to migration in an explanation afterward.
2. Kind of a migration.
( 1 ) dendric migration
A metal deposits for branch formed as in a typical occurrence shape of a migration, going to an anode from a cathode. At almost case, occurrence of a dendrite can be observed only on the surface of the insulator. This phenomenon is seen with a component of IC and a printing wiring board.
( 2 ) colloidal migration
A migration occurs in the state that a metal penetrates an insulator from an anode. This case was observed that indications depending on a metal oozed out from anode at a direction where isn't an electrode. This migration is seen with a printing wiring board and a terminal.
( 3 ) migration inside insulator.
If resin is used as an insulator for protective film, a metal spreads for cloud formed inside the resin. A metal thrusts and breaks a film and also reaches a surface, and damaged to the electronic circuit. This type occurs from an anode side mainly.
( 4 ) CAF ( Conductive Anode Filament ).
A metal deposits at surface of a fiber inside a printed circuit board. This deposit becomes a problem at a paper phenolic board and glass epoxy board using the through hole. This type occurs from an anode mainly.
3. Mechanism of a migration.
( 1 ) migration of silver.
In order to investigate a mechanism of an occurence of migration phenomenon of silver, paper was keeped between electrodes of two silver and left for 6 hours under an electrode interval is 12.5 mm, difference of potential is 45 V, condition of a relative humidity is 98 % RH. As the results of experimental, dendrite of silver that grows as going to an anode side from a cathode, and colloid of silver deoxide ( Ag2O ) to go to a cathode was observed from an anode side. Then, a mechanism of this migration is explained as follows.
① Ag is ionazed by H2O which was absorbed to surface from ambient moisture and applieing electrical potential between electrodes.
Ag → Ag+
H2O → H+ + OH-
② Ag+ and OH- deposit as generating AgOH on the side of an anode.
③ Ag2O decomposed from AgOH is dispersed for colloid on the side of anode.
2AgOH = Ag2O + H2O
④ after this, by a hydrate reaction,
Ag2O + H2O = 2AgOH = 2Ag+ + 2OH-
As the reaction is advanced, silver ion moves on the side of a cathode and a deposit of dendrite formed of silver is advanced.
As the barrier was deposited for Sb2S3 on a paper between silver electrodes, the dendrite of silver was not growing in territory of an anode, in order to catch silver ion and to convert for another material. Further, study of using Ag-Pd alloy, progressive speed of a migration became slow, also was reported.
( 2 ) migration of copper.
Migration of copper is similarly to silver. If electrical field and water exist, the following reaction comes with an anode.
Cu + 4H2O → Cu(OH)2 + O2↑ + 3H2↑
Further, with a cathode,
Cu(OH)2 → CuO↓ + H2O
Progressing these reactions, the dendrite of CuO grows as going to an anode side from a cathode side.
Anarizing examples by EDX , which shorted circuit depending on an insulation failure of the printing board to occur at a field, the distribution of Cu and Cl deposit coincided well. From this result, Cl is related to the migration of Cu. The following reaction comes with an anode.
Cu + Cl- → CuCl + e-
Cu + 2Cl- → CuCl2 + 2e-
Cu2O + 4HCl → 2H[CuCl2] + H2O
H[CuCl]2 → H+ + [CuCl2]-
[CuCl2]- → CuCl2 + e-
CuCl → Cu+ + Cl-
CuCl2 → Cu2+ + 2Cl-
On the other hand, with an anode,
Cu+ + e- → Cu
Cu2+ + 2e- → Cu
As Cl- is recycled at all times by these reactions, even though the absolute quantity to exist is scant, it is estimated that a migration of copper makes a progress of the serial.
( 3 ) other migrations.
A report of a metal migration outside silver and copper is performed. Field examples of gold, nickel, tin, lead and solder are reported, but the detail of mechanism does not understand well as yet. The order of which a migration can easily occur is Ag > Pb ≧ Cu > Sn > Au , in distinction from order of an electric chemical line of ionization. Migration does not occur for Fe, Pd and Pt. There is not a clear explanation that a metal becomes such an order to be able easily to occur for a migration.
4. Experimental method of a migration.
( 1 ) temperature and humidity.
・Test to use an environment chamber.
It is a method to use an environment test machine to obtain a stable temperature and a stable humidity. Though air is circulated, there is the case that it accompanies an error, because humidit control is not easily. Especially, a caution is necessary in case of high humidity.
・Saturated salt solutions method.
This method is test of migration in a sealing case included over saturated salt solutions. Though a temperature and humidity can set up it correctly, an influence of an element that is included in salt is been anxious about.
・Water drop method
Tests of migration at a humidity ( gas ) need a long time at many cases. In order to shorten experimental time, super-oure-water is droped between electrode, and migration is evaluated. This method of migration is used as an evaluation method in a short time, though it is not clear that a mechanism depending on a humidity is same as this.
・Temperature cycle test.
By repeating a different temperature and humidity state, nearly useing situation with an actual field, it is tried to evaluate a migration. Repeating a cycle test, an occurrence of a migration will be early, but detailed example has not been reported yet.
Further, as a more humid state, a test of a migration is performed in dewfalled state. However, similarly to a water drop test, a relation with an occurrence mechanism of a migration depending on a moisture isn't clear.
5. Evaluation method of a migration.
( 1 ) optical and electronic observation
・Optical microscope.
Most of migration phenomenon confirms as seeing in an eye. By an optical microscope observation, a comparison with a normal sample is practiced. Not only a superficial observation of a sample, a section of a sample is observed also. A shape of a migration, growth speed and a color of a dendrite are measured with an optical observation.
・Scaning electron microscope (SEM)
SEM is used in order to investigate the metal micro situation deposited on an insulator by a migration. Further, with many SEM observations, a composition is analysis at the same time. However, a superficial observation of an insulator is possible, and an insulator inside observation isn't possible.
( 2 ) composition analytic method.
・EDX, EPMA, and ESCA
About metal component and the chemical compound, the composition and quantity are measured using by these equipments. In these equipments, as being able to measure an existent map of an element, it can be confirmed whether an electrode metal maigrated actually.
・FTIR.
By FTIR of a reflection type, the organic components of migrated part is anlized. There is a report that a metal complex has an influence on a migration, so an analysis of organic matter is a valid measure for future research.
・Gas analysis.
Analyze inside gas of a hybrid IC of a sealing up case is valid to investigate a relation with a gas ingredient, including a moisture, to migration.
( 3 ) electric measurement
・Measurement of resistance
If a migration occurs, a many cases resistance value of insulator falls down. This method is measurement of electric variation from a current value that flows for an experimental circuit. Time of a trouble, which is time intarval as measurement value is lower than the decided value in advance, is useful for an evaluation of a migration.
6. An influence of an environment, and a prediction of life
( 1 ) relation of a migration and an electric field.
Time of failure expressed experimentally as a function of electric field and can be written as:
tr = ke E-n
Where, tr is a time of failure ( h ) , ke and n are constant , E is an electric field ( V/mm ). For silver on the phenolic board, n is about 0.4.
( 2 ) relation of a temperature and a migration.
Time of failure expressed experimentally as a function of absolute temperature and can be written as:
tr = kT exp ( ΔQ / RT )
Where, tr is time of failure ( h ), kT is constant, ΔQ is an activation energy, R is the gas constant. An activation energy of silver on the phenolic boad is an about 0.2 eV.
( 3 ) relation of a humidity and a migration.
Time of failure expressed experimentally as a function of humidity and can be written as:
tr = kh H-n
Where, tr is time of failure ( h ), kh and n are constant, H is a humidity ( % R.H. ) . As a case of silver on a phenolic board, n is 8 to 17 by the report, but it isn't clear as yet. A humidity is an environment condition to give the most large influence to an occurrence of a migration.
7. Future problem.
In research by today, though considerable thing understood it about a migration, a clear answer isn't obtained to prevent a migration. In a field, though an occurrence of a migration is fact, wheter it occured in any environment is indistinct and can't be reproduced. In a laboratory, it is difficult to maintenace of a stable condition and to reproduce an experiment presisely by an existence of impurities. In order to prevent a migration, a future theme that must solve is describe as follows.
( 1 ) Mechanism of a migration
It is well known that a migration occurs by electric chemical phenomenon. But, a deposit shape of a dendrite observed by this phenomenon is not clear. A mechanism of a migration to occur from an anode also isn't clear. As an occurrence mechanism of a void and a hillock to occur inside an electrode of an anode isn't a large difference of potential, the cause is indistinct.
( 2 ) electrode metal.
Like Ag-Pd alloy, by changing an electrode composition, there is a report to prevent a migration. However, the report that I investigate isn't about a relation of a metal crystal and structure and migration. A method to prevent a migration by an alloy is powerful as a method not to make an electric characteristic bad. If a why silver can easily occur for a migration the most physical explanation is performed, then, a development of a material not to occur for a migration is easy.
( 3 ) reliablity to test a migration.
In a laboratory at a test of a migration, a problem is that a measurement data isn't stationary. Even though testing condition is similar and useing a similar sample, an occurrence state of a migration differs. Fluctuating of an environment state, an existence of an impurities, difference of a composition and shape of a sample is a cause. However, it is virtually impossible to measure an influence of these individual elements. It isn't clear about whether taking out these element is not possible at an experiment, and whether a small difference is amplified as a result. An establishment of a standard experimental method is early hoped.
( 4 ) prediction of life
A product that is offered in a field, has product life inevitably. A product is manufactured as estimating the life in advance. Migration phenomenon very shortens for this product life. There, an establishment of prediction expression of life depending on this migration is demanded.
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