PCB reliability ESPEC Technology Report No.58 1 Technology Report Humidity Resistance Evaluation of Adhesive Interface, Between Conductive Adhesive and Plating Takuya Hirata, Hirokazu Tanaka ESPEC CORP. Technology Development Div. onductive adhesives have outstanding features that enable use in low-temperature packaging and heat-resistant packaging able to withstand temperatures over 150°C. While these features are making conductive adhesives an increasingly popular packaging technology, they are subject to open failures caused by deterioration of their electrical conductivity with tin-plated components under high-humidity environments. This failure mode is peculiar to conductive adhesives, but there are no set methods or test conditions used to evaluate it. To address this issue, we developed an evaluation pattern and failure detection method enabling detection of open failures caused by high-humidity environments. We used them to evaluate the humidity resistance of conductive adhesives with various types of plating, and to investigate the humidity acceleration factor of components with tin plating showing conductivity deterioration. This report presents our findings. This report is a revised version of a presentation given at the 21st Autumn Reliability Symposium held by the Reliability Engineering Association of Japan in October 2008. Conductive adhesives are a promising technology for applications in fields such as heat resistance and low-temperature packaging. Conductivity deterioration happening under humid-environment is dependent on the adhesive’s compatibility with the adhered material, and it is an issue encountered with the commonly used silver (Ag)-based conductive adhesives. 1 Open failures caused by humid environments when conductive adhesives are used on tin- (Sn-) plated parts are different from the failures encountered in soldered joints, and are a key issue when considering the reliability of conductive adhesives. However, the characteristics of conductive adhesives in humid environments and the relationship between stress and failures are largely unknown, and effective test conditions for explicating these areas have not been set forth. C Introduction 1
Transcript
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ESPEC Technology Report No.58
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Technology Report
Humidity Resistance Evaluation of Adhesive Interface,
Between Conductive Adhesive and Plating
Takuya Hirata, Hirokazu Tanaka ESPEC CORP. Technology Development Div.
onductive adhesives have outstanding features that enable use in low-temperature packaging and heat-resistant packaging able to withstand temperatures over 150°C. While these features are making conductive adhesives an increasingly popular packaging technology, they are subject to open failures caused by deterioration of their electrical conductivity with tin-plated components under high-humidity environments. This failure mode is peculiar to conductive adhesives, but there are no set methods or test conditions used to evaluate it. To address this issue, we developed an evaluation pattern and failure detection method enabling detection of open failures caused by high-humidity environments. We used them to evaluate the humidity resistance of conductive adhesives with various types of plating, and to investigate the humidity acceleration factor of components with tin plating showing conductivity deterioration. This report presents our findings. This report is a revised version of a presentation given at the 21st Autumn Reliability Symposium held by the Reliability Engineering Association of Japan in October 2008.
Conductive adhesives are a promising technology for applications in fields such as heat resistance and low-temperature packaging. Conductivity deterioration happening under humid-environment is dependent on the adhesive’s compatibility with the adhered material, and it is an issue encountered with the commonly used silver (Ag)-based conductive adhesives.1 Open failures caused by humid environments when conductive adhesives are used on tin- (Sn-) plated parts are different from the failures encountered in soldered joints, and are a key issue when considering the reliability of conductive adhesives. However, the characteristics of conductive adhesives in humid environments and the relationship between stress and failures are largely unknown, and effective test conditions for explicating these areas have not been set forth.
C
Introduction 1
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For our evaluation, we used a standard Ag-based conductive adhesive to develop an evaluation pattern and failure detection method able to detect open failures caused by high-humidity environments. Under various humid-environment conditions, we examined the changes in the resistance values of connections with various types of plating, and made cross-sectional observations of these connections. We studied the moisture absorption characteristic of conductive adhesive to extract the causes of open failures. We also investigated the humidity acceleration factor of Sn-plated connections showing conductivity deterioration. Our findings are presented here.
To identify the humidity stress factors of isotropic conductive adhesive paste (shortened below to ‘conductive adhesive’), we developed simple connection model test boards consisting of conductive adhesive and the adhered material. Table 1 lists the test samples and test conditions we used. Conductive adhesives can be of several compositions depending on the application. The type we used for testing was a standard Ag/epoxy-based conductive adhesive with no special additives. Sample Conductive adhesive
Figure 1 illustrates the test boards used. Each test board had one of the three types of surface treatment—copper (Cu), Cu + Sn plating, or Cu + gold (Au) plating. onto make the conductive adhesive adhere to insulators, we used the screen printing method that electrically connected the wiring patterns with conductive adhesive on a glass epoxy board (FR-4). We carried out temperature/ humidity testing under a total of 5 conditions, using a temperature of 85°C and relative humidity of 85% as the reference conditions (Table 1). Figure 2 illustrates how we measured resistance to detect conductivity deterioration. We measured resistance by the four-terminal method, taking measurements every 3 minutes in a temperature/ humidity environment (Figure 3). After testing, we made cross-sectional observations of the test boards using a scanning electron microscope and electron probe microanalyzer (EPMA).
3.1 Resistance changes and failure distribution Figure 4 shows the resistance changes during the temperature/ humidity tests. In the 85°C/85% test environment, we found differences in electrical characteristics linked to differences in board surface treatment (Figure 4-a). We found that the resistance value of the Sn-plated test board increased and its failure mode was open failure. The Cu and Au plated test boards had no change in resistance value throughout 500 hours of testing.
Results and Conjectures 3
Fig. 2 Measurement method of the conductor resistance to detect conduction
degradation
Fig. 3 Example of test system (Temperature & humidity chamber
combined with a Conductor Resistance Evaluation System)
Fig.1 Test board to evaluate conductive adhesives with various plating surfaces
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Their resistance values remained stable, resulting in no open failures. The results below therefore pertain to the Sn test board, which we subjected to data analysis and investigation to identify failure factors. Figure 4-b shows the change in resistance during each temperature/ humidity test. We found a greater increase in the resistance value under conditions of high temperature and relative humidity, and by comparing two sets of conditions (60°C/85%rh and 85°C/60%rh), we found that conductivity deterioration does not depend on the vapor pressure. We set a resistance value of at least twice the initial value as the failure criterion, and adapted the distribution to a Weibull distribution.
Fig. 4 Conduction resistance changes under temperature & humidity test
Figure 5 shows the Weibull plot of conductivity deterioration. Under each set of temperature/ humidity conditions, the shape parameter of the Weibull distribution is nearly the same, and we conjecture that the failure mechanism is also the same.
b) Comparison of boards’ surface treatment
(85ºC/85%rh test)
a) Comparison of test conditions
(Sn plating)
Fig. 5 Breakdown distribution of Sn plating where open breakdown occurs
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3.2 Failure analysis to determine failure mechanisms To investigate the causes of conductivity deterioration, we made cross-sectional observations of the test samples after testing. Figure 6 is a scanning electron micrograph of an Sn plating connection interface after 500 hours of temperature/ humidity testing. It suggests the presence of Sn oxidation at the connection interface between the conductive adhesive and Sn plating after the test.2 We confirmed this Sn oxidation using an EPMA to perform element analysis of the cross-section (Figure 7). We found no Sn oxidation on tested samples with only Sn-plated wiring, possibly indicating that conductive adhesive components contributed to Sn oxidation.
Fig. 6 SEM image of conductive adhesive and Sn plating adhesive interface
3.3 Moisture absorption volume in each test environment To study the factors responsible for Sn oxidation in temperature/ humidity environments, we measured the volume of moisture absorbed by the conductive adhesive. We measured the volume of moisture absorbed by the conductive adhesive
a) Start b) After testing (85ºC/85%rh, 500 hrs)
Fig. 7 Elements analysis results of conductive adhesive and Sn plating adhesive interface (after 85ºC/85%rh, 500 hrs testing)
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alone, formed into test samples of 50 × 50 × 1 mm in size. The test samples were created by heat treatment for 30 minutes at 150°C. After drying the samples for 24 hours at 50°C as a preprocessing procedure, we placed them in the same temperature/ humidity test environments shown in Table 1. We removed them at preset intervals to measure their changes in weight with a precision balance, and calculated the conductive adhesive’s moisture absorption ratio. 3 Figure 8 shows the moisture absorption characteristic in each temperature/ humidity environment. The conductive adhesive’s moisture absorption curve resembles the general characteristic for polymers. Since the relationship between each set of test conditions and the moisture absorption volume resembles the electrical deterioration relationship, the volume of absorbed moisture may affect Sn oxidation. We conjecture that the moisture absorbed into conductive adhesive contributes to phenomena such as free ion diffusion in the epoxy resin, electrical field formation between Sn and Ag, and Sn ionization. 1
3.4 Humidity acceleration factor for Sn plating conductivity deterioration We used the Sn plating failure distribution to study the acceleration factor for our humidity
resistance testing. We created an Arrhenius plot using the median life estimated from the
Weibull distribution parameters (Figure 9). The plot suggests that Sn plating oxidation is
temperature-dependent at a condition of constant 85% relative humidity. We estimated the
activation energy at 0.78 eV.2
We then studied the humidity acceleration factor by trying to adapt it to an acceleration
model formula. We found it matches the Eyring temperature/humidity dependence model4
0.0
0.1
0.2
0.3
0.4
0.5
0 20 40 60 80 100
時間 （h）
吸湿
率
（％）
60℃/85％
75℃/85％
85℃/60％
85℃/85％
Fig. 8 Moisture absorption characteristic of conductive adhesive
Time (hr)
Moisture absorption
ratio (%)
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0
1
2
3
4
5
6
7
2.7 2.8 2.9 3 3.1
1000/T (K-1)
ln L50 (h)
0
2
4
6
8
31.5 32.5 33.5 34.5 35.5
(Ea/κ T)+(B/RH)
ln L50 (h)
shown in Formula (1) (Figure 10):
L ∝ exp Ea
kT ⋅ exp BRH (1)
where Ea is the activation energy, k is the Boltzmann constant, T is the absolute temperature,
B is a constant, and RH is the relative humidity. From our test results, we estimated that
activation energy Ea is 0.78 eV and constant B is 605.
The results presented indicate that temperature and relative humidity contribute as acceleration factors to Sn plating oxidation failure of conductive adhesive packages. Of the test conditions we used, environments of 85°C and 85% relative humidity resulted in the highest acceleration factor. We used test boards with Sn-, Cu- and Au-plated wiring patterns to evaluate the humidity resistance of conductive adhesive. We examined the changes in resistance during testing, analyzed the cross-sections of failure sites, studied changes in moisture absorption volumes, and investigated the humidity acceleration factor. We made the following findings: (1) Failures due to the humid environment at the interface between the conductive adhesive and various types of plating were significant for Sn plating. The failure mode was open failure caused by an increase in resistance value. (2) Our failure analysis found that oxidation of the Sn-plated adhesion interface was the factor responsible for resistance value increases. (3) The moisture absorption characteristic of conductive adhesive is similar to the moisture absorption characteristic of polymers—moisture absorption increases in proportion to temperature and humidity.
Conclusion 4
Fig. 10 Temperature & humidity acceleration of Sn plating conduction degradation
Fig. 9 Temperature acceleration of Sn plating conduction degradation
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(4) We conjecture that the stress factors that contribute to conductivity deterioration of conductive adhesive in a humid environment are temperature and relative humidity. Deterioration time decreases in proportion to increasing temperature and humidity. Acknowledgments This research was carried out as part of a NEDO (New Energy and Industrial Technology Development Organization) project, R&D of Alternatives to High Temperature High Lead Solder, and received some assistance from that project. The authors would like to express their heartfelt appreciation to all involved. Bibliography 1. R&D of Alternatives to High Temperature High Lead Solder Project (2008): “Presentation Materials on Results of R&D of Alternatives to High Temperature High Lead Solder” [in Japanese], New Energy and Industrial Technology Development Organization 2. Takuya Hirata, Yasutoshi Nakagawa and Hirokazu Tanaka (2008): “Temperature humidity testing and evaluation of isotropic conductive paste on circuit boards, [in Japanese], Collection of Conference Papers from 18th Micro Electronics Symposium, pp. 183 to 186. 3. JEDEC Standard JESD22-A120A (2008), “Test Method for the Measurement of Moisture Diffusivity and Water Solubility in Organic Materials Used in Electronic Devices” 4. Lycoudes, N.(1978) : “The reliability of plastic microcircuits in moist environments,” Solid St. Technol., Vol.21, pp.53-62
