Karthik Vijay, Technical Manager, Europe, Africa, and the Middle East Indium
Presented by Karthik Vijay, Technical Manager, Europe, Africa, and the Middle East
Karthik is based in the UK and manages Indium Corporation’s technology programs and technical support throughout Europe. His expertise is focused on solder paste, engineered solders, thermal interface materials, and semiconductor-grade electronics assembly materials. Karthik is active in several industry organizations, including IMAPS and the Surface Mount Technology Association (SMTA), and has presented at industry forums and conferences internationally. He earned his master’s degree in systems science and industrial engineering from Binghamton University, State University of New York.
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SIR test using glass slides best mimics electrical reliability of fluxes for low-standoff components. Of all the fluxes tested, only Indium Corporation’s flux passed SIR requirements. This solder paste exhibited a SIR value well above the IPC spec 100 MΩ without any dendrite formation, even with a wet flux residue on the comb pattern.
Indium Corporation’s flux also achieved very good wetting, low voiding, and HIP resistance. Achieving enhanced electrical reliability without compromising on other key SMT metrics requires unique flux technologies.
Automotive Electronics – Dendritic Growth and Corrosion Under Low-Standoff Components: A Flux Solution
Low-standoff components, such as double-MOSFETs and power QFNs in automotive electronics, are proliferating in automotive electronics for reasons of cost and functionality – soldering them with solder paste is the most cost-effective option in high-volume PCB assembly.
However, due to poor venting, no-clean flux volatiles are not able to outgas sufficiently and dry off. This wet flux residue causes dendritic growth and corrosion for demanding electrical reliability requirements in harsh conditions such as (a) high heat exposure at 120°C for 500 hrs, 140°C for 500 hours, 30V bias (transmission control); and (b) exposure to heated oil at 85°C for 500 hrs (braking unit).
For a no-clean system where the flux residue is benign, the low standoff entraps solvents, the flux residue becomes liquid or semi-liquid, and activators can move around resulting in a corrosive reaction – particularly with humidity and bias conditions.
To prevent dendritic growth, one option is to remove the flux residue post reflow. However, this can be challenging since small venting spaces are barriers for cleaners to remove the flux residue. In addition, a cleaning step adds costs (equipment and cleaners) and extra process steps. The second option is to use a flux that is non-corrosive and non-conductive, even if drying of flux residue is not possible. In other words, develop a flux that is benign, regardless of whether it is wet or dry.
|A||Halogenated, no-clean, T4|
|B||Halogenated, no-clean, T4.5|
|C||Halogen-free, no-clean, T4|
|D||Halogen-free, no-clean, T4|
|Indium||Halogen-free, no-clean, T4|
A wet flux residue that is non-corrosive and non-conductive should contain very low amounts of activators and exhibit very low activity. However, this type of flux would be poor in wetting, which in turn would be poor in voiding [1, 2] and HIP performance , hence not be adequate for SMT assembly applications. After extensive research and development efforts, Indium Corporation’s halogen-free, no-clean flux was developed to eliminate dendritic growth and corrosion, and at the same time achieve good wetting, low voiding and eliminate HIP.
To prove its effectiveness, six solder pastes were tested, including Indium Corporation’s solder paste and five conventional solder pastes as controls. The characteristics of those six solder pastes are shown in Table 1. All solder pastes employed 96.5Sn/3.0Ag/0.5Cu (SAC305), type 4 (20-37 microns) or type 4.5 solder powder. Pastes A, B, and C are standard products well-received by the market as SMT no-clean lead-free solder pastes. Pastes D and E are commercially-available materials; the powder size of E was not available.
Karthik Vijay, Indium Corporation’s Technical Manager, Europe, Africa, and the Middle East will be on hand at Innovations Forum on June 16th in Budpaest, Hungary. Among the topics that Karthik will cover are the results of testing.
Surface Insulation Resistance (SIR) Test
A standard IPC B-24 SIR board was used for this experiment. The flux vehicle of each of the solder pastes was printed onto three of four comb patterns in the SIR comb pattern using a 0.10 mm (4 mil) thickness stencil. A 10 mm × 10 mm glass slide was placed onto a part of the comb pattern printed with flux. Two comb patterns received this glass slide coverage treatment, as shown in Figure 1. Each of the glass slides was further secured onto the SIR coupon with 3M high-temperature tape to avoid slide movement during subsequent air reflow. The reflow was conducted via a convection oven with a peak temperature of 244oC, with the profile shown in Figure 2. Other than the coupon preparation, the SIR testing was performed in accordance with J-STD-004B. The SIR performance of the fluxes on SIR coupons without glass slide coverage (standard SIR test) was also evaluated for comparison purposes. Here, all flux fumes vaporized with dry flux residue left behind on the comb pattern.
It should be noted that during the preliminary trials, solder paste was printed onto comb patterns followed by placing glass slides. After reflow, the flux residue under the glass slide was found to be dry, despite the glass slide coverage. This was caused by too high a standoff when the solder rim formed on the comb pattern. In order to mimic the hampered venting of flux fumes, printing the flux vehicle instead of solder paste was found to be effective and resulted in wet flux residue.
- Wanda Hance and Ning-Cheng Lee, “Voiding Mechanisms in SMT,” China Lake’s 17th Annual Electronics Manufacturing Seminar, 1993.
- Ning-Cheng Lee, “Reflow soldering processing and troubleshooting SMT, BGA, CSP, and Flip Chip Technologies,” Newnes, pp. 288, 2001.
- Yan Liu, Pamela Fiacco, and Ning-Cheng Lee, “Testing and Prevention of Head-In-Pillow.” Surface Mount Technology Association (SMTA) China South Conference at NEPCON Shenzhen, August, 2010.
Indium Corporation is a premier materials manufacturer and supplier to the global electronics, semiconductor, thin-film, thermal management, and solar markets. Products include solders and fluxes; brazes; thermal interface materials; sputtering targets; indium, gallium, germanium, and tin metals and inorganic compounds; and NanoFoil®. Founded in 1934, Indium has global technical support and factories located in China, Malaysia, Singapore, South Korea, the United Kingdom, and the USA. For more information about Indium Corporation, visit www.indium.com or email email@example.com. You can also follow our experts, From One Engineer To Another® (#FOETA), at www.facebook.com/indium or @IndiumCorp.