Mario Reiter, senior engineer, Fraunhofer ISIT
Ionic impurities can lead to electrochemical migration (ECM) in the presence of potential differences under the influence of moisture and thus ultimately to failure of electronics. In particular, the application of higher voltages in the field of power electronics and increasingly also in the automotive industry places high demands on system quality and reliability. In order to provide another building block for reliability assessment, surface insulation resistance (SIR) measurements are performed at Fraunhofer Institute for Silicon Technology (ISIT) using a SIR testing printed circuit board (PCB) with 10 standard combs. Up to 80 channels can be measured in parallel as standard.
Soft soldering processes for mechanical fixation and electrical connection of components on PCBs are still indispensable in electronics production. The demands on the technology and quality of soldering processes have been increasing significantly for several years. Despite increasing integration density, the assemblies must be exposed to increasingly harsh environmental conditions. Reliability often has to be guaranteed over a long period of time, e.g., in automotive electronics. Quality requirements that used to be limited to special applications and niche markets must now be implemented under the economic constraints of mass products.
The chemical reactivity of fluxes is necessary to achieve optically and also functionally flawless solder joints. However, this reactivity can endanger the reliability of the product in the medium to long term. The essential effect here is ECM: the ions remaining on the circuit board become mobile through the action of moisture and attack the metallic structures. Due to potential differences, i.e. in any electronic circuit, successive detachment and migration of metal ions leads to the growth of so-called dendrites. These are tree-like structures that can grow from one contact to the next, eventually causing short circuits. This process can take place within a few seconds or over significantly longer periods of time.
Meaning of standardised comb features
Ever smaller PCB structures favour the underlying failure mechanisms. For the user, consequences such as a product recall or product liability can be fatal.
Other types of contamination can arise from improper handling and storage, e.g., fingerprints (grease, salts), dust from the working environment, organic deposits, etc. Substrate supplier quality also plays a role here. Indirectly, any form of surface contamination can promote ECM, e.g., by reducing the adhesion of a protective coating and thus allowing moisture to penetrate.
Even without ECM, an electronic circuit can be functionally compromised if the surfaces of the assembly are not free of residue. Highly amplifying circuits, e.g., operational amplifiers, can already be caused to switch undesirably by leakage currents in the µA range.
These examples show that SIR plays an important role in assessing the long-term reliability of an electronic assembly.
Solder paste manufacturers and industrial users consider the measurement of SIR on standardised comb structures to be a suitable tool for assessing the long-term chemical stability of the assembly. Fraunhofer ISIT uses a SIR testing PCB with 10 standard combs to perform the corresponding measurements. Up to 80 channels can be measured in parallel as standard.
A surface insulation resistance (SIR) test printed circuit board (PCB) with 10 standard combs, IPC-B-24, according to the IPC-TM-650 Test methods manual (ANSI/J-STD-004/05)
Test conditions
The test conditions are specified in the international standards IPC-J-STD-004C: Requirements for soldering fluxes and IPC-J-STD-005A: Requirements for soldering pastes.
According to the IPC-TM-650 Test methods manual (ANSI/J-STD-004/05), the surface resistance test is performed under the following conditions:
Bias voltage: 45-50VMeasuring voltage: 100VClimate: 85 percent relative humidity (rLF), 85°CTest duration: 24 h, 96 h, 168 h
A minimum SIR value of 100 megohms (МΩ) is prescribed as the limit value for the entire duration. The test conditions are often also modified by customer requirements to the corresponding application (e.g., use of lower voltages such as 24 V or 5 V; use of other climates, e.g., 92 percent rLF, 40°C or climate change stresses).
The fluxes/solder pastes to be tested may behave very differently with regard to the absolute value of the SIR or with regard to SIR curve progression (specifically the tendency of different fluxes of the SIR curve to fall, remain constant or rise).
Quality assurance of the comb structures
After the climatic test, the ridge structures are examined for changes (especially dendrite formation). Growth of dendritic structures is not allowed and is therefore a rejection criterion.The standard SIR test is usually performed on the unpopulated board to describe the properties of the solder (specifically the flux in various solder configurations: solder paste, wave solder, solder wires). Furthermore, on unpopulated boards, it is also possible to check the quality of cleaning processes or the effectiveness of passivation processes (e.g., covering by protective lacquer layers) by means of the SIR test. The measurements described qualify the long-term properties of the soldering, cleaning and lacquering media under climatic stress.
A comb structure (hot air solder levelling (HASL)) with clear dendrite formation after an SIR test.
Not taken into account here is the influence of component geometry (distances, gaps, capillary effects, etc.), real local flux quantity and distribution or influence of secondary materials (component surfaces, topography, interactions, impurities, etc.). To this end, the SIR test structures can be directly adapted to the footprint design of the components or positioned directly under components. Standardised SIR test PCBs are offered (e.g., IPC B-36, IPC B-52) for this purpose.
Mario Reiter,
senior engineer Fraunhofer ISIT