Characteristics to be measured to ensure quality
Since it is virtually impossible to re-work on a PCB that has been manufactured, the process qualification plays a major role in ensuring a reliable PCB. Process qualification is basically an assurance of quality in manufacturing processes and testing methods. It confirms that products with pre-determined quality characteristics and attributes can be reproduced consistently within the established limits of the manufacturing process.

Mechanical aspects. It is important to measure the length, breadth and thickness of PCBs while designing them. Pitches are verified for mounting because once the components are mounted, the mechanical characteristics of the PCB cannot be altered. Warp and twist play an important role during assembly. Flexural strength, bond strength, peel strength, trace width, thickness, resolution, protective coatings (solder mask, solder coating, gold plating, etc), dielectric thickness, etch back, barrel regularity and annular rings are the other aspects that are verified before subjecting PCBs to environmental tests.

Electrical aspects. Intra-layer and inter-layer insulation checks are done because up to 14 layers are qualified for space applications. Hence, the intra-layer and inter-layer insulation resistances, dielectric withstanding capability and interconnection resistances play an important role. The current carrying capacity is another important characteristic to be measured.

After mechanical and electrical checks, the PCB is subjected to thermal shock cycling, solder-float tests and vibration tests. Long-term damping for humidity verification is done because the launch stations that the spacecraft are shifted to are typically located very close to the sea shore. The PCBs have to be well protected from the humidity point of view too. Hot and cold storage of PCBs is done, followed by vibration checks again. Lastly, a hot oil dip is done to find out if there is any lamination damage due to hot oil.

Some tests performed on spacecraft PCBs

Some of the tests that are performed on PCBs to ensure their high reliability are mentioned below, with a brief explanation on the purpose of each test.

Thermal shock cycling. The purpose of this test is to determine the resistance of a material, laminate or multi-layer board by repeated exposure to extreme temperatures for comparatively short periods of time. Interconnection resistance (ICR) is measured prior to and after thermal shock cycling. ICR must not change by more than 10 per cent of the initial value. Destructive tests like micro-sectioning are done to see whether the barrel or the plated through-hole is intact or not.

Thermal stress tests. This test is done to determine whether plated through-holes can withstand the thermodynamic effects of extreme heat to which they may be exposed during the assembly rework or repair process. The PCB is ‘baked’ at 125 to 150 degrees centigrade for six hours, after which it is subjected to six dips in the molten solder at 288 degrees centigrade, with a 120-second gap between the dips.

Vibration tests. This is an important event in the testing of PCBs because during vibration, anything can happen to the PCB. The interconnections are made through copper barrels that are coated inside the holes by chemical procees. Chemically deposited holes must be intact even after they are subjected to thermal cycling and vibration tests. The PCB is clamped on the ‘vibration table’ and sine input is swept ranging from 1 Hz to 2000 Hz. The resonance frequency is determined by mounting the transducer at the centre of the PCB. At the resonant frequency, the PCB is continuously vibrated for 30 minutes, after which the critical bare board test and micro-section is performed to observe any deformation in the barrels.

Humidity storage tests. This is to determine the effect of storage in moisture, which could result in a reduction in the performance of the PCB.

Other tests that are performed are out-gassing, dielectric withstanding voltage tests, insulation resistance tests, interconnection resistance tests, hot oil dip tests, peel strength tests, network simulation, bond strength tests, bare board tests and visual inspection.

An overview of the test pattern used for achieving high-reliability PCBs at ISRO

ISRO has an in-house PCB manufacturing plant that is called a ‘protoshop.’ For testing, 11 samples of a PCB are submitted to the quality assurance division, where engineers conduct Group-1 to Group-7 tests on them. These tests reveal the quality of the processes employed in fabricating the PCBs. Also, the tests check whether the process identification documentation and flow-chart has been made in a proper manner, whether non-conformance management has been recorded properly, and whether feedback from various in-line quality control (QC) at each stage of manufacturing has been recorded. After all the tests, the manufacturer of the PCB gets the information from ISRO’s QC.

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