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Reliable electronic assemblies

Surface Insulation Resistance (SIR) measurement of jetted solder paste
Reliable electronic assemblies

Electronic assemblies grow more and more complex. The need to integrate increasingly functions into even smaller systems reduces the isolation distances in such a way that the risk of electrochemical corrosion increases critically. Climatic conditions such as high temperatures and humidity as well as temperature changes, which can lead to condensation, often have a damaging effect on the operation of electronic assemblies.

Helge Schimanski, Fraunhofer ISIT, Itzehoe (Germany)

The interaction of moisture, different electrical potential and ionic contamination may lead to corrosion and electrochemical migration. In particular, residues of fluxes, which are typically left on the circuit board surface and components after soldering, are often hygroscopic and thus promote electrochemical migration or corrosion. The result is reduced isolation resistance on the circuit board surface that might cause short circuits and ultimately leads to irreversible damage to the system.
The target of electronic assembly production is to produceacceptable soldering quality. To achieve this, it is important to apply to each single joint the correct amount of solder paste. In case of closely adjacent component terminations with greatly different requirement of solder paste volume,this presents a big challenge for solder paste application. Additive solder paste deposition might be necessary. This additional solder paste can be very effectively applied by use of jet printing. However, the question arises whether the mix of various fluxes (printed and jetted solder paste flux) is harmful.
Here, SIR test results of an investigation of flux mixtures are presented. Solder paste is applied first by stencil printing,then necessary additional solder paste is jetted. An interesting aspect in this case is that both solder pastes are delivered by different manufacturers and thus even have different flux mixtures. Both considered alone are so-called “No Clean” products, but what happens when the two fluxes are mixed and reflow-soldered?
Implementation
The Surface Insulation Resistance (SIR) Test is a method to characterize fluxes by determining the degradation of electrical insulation resistance of rigid printed wiring board specimens in the presence of moisture. The test is carried out according to IPC J-STD-004B referring to IPC-TM-650 2.6.3.3 (Surface Insulation Resistance, Fluxes) on standardized comb testpatterns according IPC-B-24 (Surface Insulation Resistance Test Board), where an individual comb structure has defined lines and spacing. The solder paste is stencil-printed first and then additional solder paste volume is jetted onto the test board consisting of FR-4epoxy-glass laminate material.
The samples run through a reflow soldering process using a lead free SnAgCu temperature profile with a peak temperature of 245°C. This reflow profile can be individually adapted on customer request.
The SIR Test is carried out at high humidity (85%rh) and heat (85°C) conditions for 168 hours. A voltage of 100V is applied to the comb structures and the resistance is measured at regular intervals. Different types of solder pastes and mixtures thereof are examined with respect to their surface resistance. As a reference, a printed circuit board with empty combs is also measured.
Evaluation and error condition
Each comb pattern on each test PCB has been evaluated by the insulation resistance values during the climate testing. If the readings at the end of the measurement periodare less than100 megohm (1E8 ohm), the test is evaluated as fail. All specimens have been examined optically within 24 hours of completing the testing. Visible discoloration, corrosion or dendritic growth, which will constitute a failure, have been reported.
Conclusion
The measured curve shows that the empty comb has almost 2 orders of magnitude higher resistance values (1E10 ohm) than required. In all the samples, a reduction of the surface resistance compared to the blank comb can be seen. There are slight uncritical differences in resistance between the solder pastes. The mixtures provide no negative effects and the measured surface isolation resistances are in the range of the base materials. All investigated solder pastes and mixtures thereof shows ufficiently good SIR test results. Complying with all tested combinations however, the limit of 100 megohm is safe.
This study covers only a small number of possible combinations of flux materials. Additional combinations should be tested if needed. The question of the flux behavior in case of repair process remains unanswered: What will happen if flux or mixtures of fluxes are heated up to undefined temperature time profiles? Ionic contamination could remain on the PCB surface and lead to corrosion and migration. This question is part of an actual investigation project.

Zusammenfassung
Die elektronischen Bauelemente werden zunehmend komplexer. Der Trend zur Integration von mehr und mehr Funktionen in noch kleinere Systeme reduziert die Entfernung der Isolationen derart, dass sich die Gefahr elektrochemischer Korrosion kritisch erhöht. Die im Artikel aufgezeigten Untersuchungen beschäftigen sich mit der Bestimmung des Oberflächenisolationswiderstands von Lotpasten.
Les éléments de construction électroniques deviennent de plus en plus complexes. La tendance à l’intégration de fonctions de plus en plus nombreuses dans des systèmes toujours plus petits, réduit la distance de l’isolation de telle manière, que le risque de corrosion électrochimique augmente de manière critique. Les examens présentés dans l’article traitent de la détermination de la résistance de l’isolation de surface pour des pâtes à souder.
Current Issue
Titelbild EPP EUROPE Electronics Production and Test 11
Issue
11.2023
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