Electronics Assembly Knowledge, Vision & Wisdom
Surface Insulation Resistance of No-Clean Flux Residues
Surface Insulation Resistance of No-Clean Flux Residues
No-clean fluxes present great benefits, but the activity of the unwashed process residues must be tightly controlled in order to meet reliability standards.
Materials Tech

Authored By:
Bruno Tolla, Ph.D., Denis Jean, Kyle Loomis, Yanrong Shi, Ph.D.
Kester
Itasca, IL, USA
,{url:'http://www.circuitinsight.com/videos/programs_final.mp4'}], clip:{autoBuffering:true, autoPlay:true, scaling:'scale' } }).ipad();
Summary
No-clean fluxes present great benefits for the Electronic assembly industry, but the activity of the unwashed process residues must be tightly controlled in order to meet high reliability standards. The pervasive miniaturization trends of the industry, coupled with a complexification of the component architectures profoundly affect the nature and the reactivity of the flux residues. A series of customized Surface Insulation Resistance Experiments under various SMT components demonstrate the dramatic impact of the partial activation of the fluxes, unevaporated solvents and non-decomposed activators on the reliability of the final assembly. Mainstream no-clean pastes and liquid fluxes, which are qualified under all the standard SIR and ECM reliability tests, present SIR values several decades lower than the 100MΩ limit mandated by IPC J-STD-004B when tested under QFNs. Different surface mount components (Passive, QFP, BGA) can be more or less forgiving depending on the induced heat gradients and resistance to outgassing. From this perspective, we demonstrate how a thorough examination of the interplay between assembly architecture, processing conditions and flux formulation is the necessary condition for the design of reliable fluxes mitigating the risks of in-field failures of the final assembly. This study forms the background for the proposal of new reliability testing standards for the electronic assembly industry.
Conclusions
This study demonstrates the strong interaction between solder materials and components in determining the reliability of an electronic assembly. The multiple mechanisms at play were described, and their dramatic impact was assessed through the comparison of simple and open structures (capacitors, BGAs) with more complex architectures (QFNs). The latter creates a challenging environment for some commercial pastes, who dramatically fail reliability tests while performing adequately under current industry standards. On the other hand, it is demonstrated that solder material suppliers are able to design robust formula performing reliably under various environmental conditions and with a large set of components.

We showed that complex leadless devices like QFNs create specific issues due to their greater thermal mass, low standoff, and the tortuosity of their outgassing channels. This architecture is prone to trap solvents and decomposition products, and also creates thermal gradients altering the complex chemical processes at play during reflow. While these processes were studied in detail, it is difficult to discriminate their impact. Regardless, the general differences observed between open architectures (BGAs, capacitors, open conditions) and QFNs indicate that the
outgassing effects are prevalent.

These results highlight the importance of the design of representative qualification protocols for electronic assemblies, in terms of architecture and end-usage environment (T, RH, Voltage Gradients). This requirement becomes critical when low stand-off components are to be used, a common trend of today's electronics industry. Consequently, there is a need to update the testing and qualification standards, and we certainly hope the customized boards presented here will participate to this reflection.
Submit A Comment

Comments are reviewed prior to posting. Please avoid discussion of pricing or recommendations for specific products. You must include your full name to have your comments posted. We will not post your email address.

Your Name


Company


E-mail


Country


Comments


Authentication

Please type the number displayed into the box. If you receive an error, you may need to refresh the page and resubmit the information.



Related Programs
bullet Reliability of Doped Lead-Free Solder Joints Under Isothermal Aging
bullet Using Solder Paste Beyond the Expiration Date
bullet Pressure-Less Silver Sintering Pastes for Low Porosity Joint and Large Die
bullet Utilizing an Alternative Solder Paste to Improve the PCB Assembly Process
bullet Excess Flux Residue After Hand Soldering
bullet Reliability of No-clean and Water-soluble Solder Pastes
bullet The Role of Organic Amines in Soldering Materials
bullet Grain Refinement for Improved Lead-Free Joint Reliability
bullet Meeting Future Stencil Printing Challenges with Ultrafine Powder Pastes
bullet Suitable Flux Medium for Cleanable and No-Clean Solder Pastes
More Related Programs