Authored By:
Jimmy Shu, Nat Govea and Westin Bent
MacDermid Alpha Electronics Solutions
Summary
No-clean solder paste is generally preferred for the assembly of Mobile, Automotive and Consumer electronics. Increased reliability requirements for these electronics means manufacturers must use materials capable of withstanding in-use conditions and meeting these requirements. Handheld portable electronics are susceptible to being dropped and must be protected against the effects of mechanical shock. Automotive electronics require protection against vibrational and thermal effects that affect reliability. Underfill and Edgebond can be used to reduce the impact of mechanical and thermal stress on reliability. Traditionally, the focus of materials suppliers has been on reporting certain material properties of their Underfills and Edgebonds.
Properties of these materials in a fully cured state, such as glass transition temperature (Tg), modulus of elasticity (E), coefficient of thermal expansion (CTE) and adhesion, are commonly reported and are used by customers to determine the suitability of a particular material for their assembly process based on reliability requirements. These material properties are very important to consider, but there are other factors that deserve our consideration as well. One such very important factor we cannot ignore, is the effect of interactions between Underfill and Edgebond materials and no-clean solder paste flux residue present on the assembly after reflow. How compatible the Underfill and Edgebond chemistries are with any flux residues present on the circuit board will ultimately determine how much of an effect these materials will have on the reliability of the final assembly.
This paper presents the methodology for evaluation of Underfills and Edgebond materials in combination with no-clean solder paste. Several crucial criteria were identified to determine compatibility of the materials: effect of solder paste flux residue on the curing of the Underfill and Edgebond materials, as well as adhesion of the materials not only on solder mask but also on surfaces covered by solder paste flux residue. The presence of Underfill and Edgebond materials in combination with flux residues increases the chemical complexity present on the board and the potential for electrochemical reliability issues. Surface Insulation Resistance (SIR) reliability of several combinations of Underfill and Edgebond with no-clean flux residues was measured assessed as per J-STD-004A (TM-650-2.6.3.3), J-STD-004B (TM-650-2.6.3.7) and ECM (TM-650-2.6.14.1).
The evaluation results showed that tested no-clean solder paste flux residue does not affect curing performance of tested Underfill and Edgebond materials. Some variation in materials adhesion strength was observed on different surfaces. For tested materials, no degradation in electrochemical reliability was observed for single materials versus combination of the materials (solder paste and Underfill and Edgebond). Recommendations for compatibility evaluation for solder pastes and Underfill and Edgebond materials were developed.
Conclusions
Compatibility of the Edgebond and Underfill materials with no-clean solder paste residue is a critical attribute of the combination in order to achieve essential mechanical and electrochemical reliability. There are several ongoing studies to develop better understanding and comprehensive selection guide for reinforcement material for specific board design and chosen solder paste.
Initially Published in the SMTA Proceedings
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