Here is a report that digs deep into the properties of automotive-grade solder paste and its use in high-power electrical systems
Increased automotive electrification – including 48 V hybrid electric vehicles (HEV) and electric vehicles (EV) with under-the-hood PCBA platforms, such as 48/12 V DC-DC converter PCBA (printed circuit board assembly) and voltage stabiliser PCBA for start-stop modules – has resulted in the prolific usage of high-power components, such as DPAKs (TO-252, decawatt package) or power-QFNs. These components are typically classified as low-standoff, meaning the gap between the PCB pad and the component underbody is typically under 75 μm. During reflow, volatiles in the no-clean paste flux chemistry, such as activators and solvents, boil off. Automotive-grade solder paste is the answer.
In the application on PCBAs, the lower standoff of high-power components reduces the opportunity for the flux volatiles to vent, potentially resulting in “wet” flux residue post-reflow. It is critical that the no-clean flux residue does not cause ionic dendritic growth and corrosion under low-standoff power components during the working life of the product. For these reasons, the automotive industry is imposing more stringent Surface Insulation Resistance (SIR) requirements on PCBAs to assess the electrical reliability of the no-clean solder paste flux chemistry, versus the standard J-STD 004B IPC-SIR test requirements. These include increased test voltages, increased test time, reduced space width, upping the minimum SIR threshold, and usage of different test coupons.
LEAKAGE CURRENT AND FAILURE OF HIGH-POWER MOSFET
In a study conducted by Indium Corporation, a power metal-oxide-semiconductor field-effect transistor (MOSFET) low-standoff component was assembled with a no-clean solder paste and subjected to 50 V, 90 % RH for 1,000 hr. In this example, ionic dendritic growth was observed that resulted in leakage current and eventual failure of the assembly. This failure was specific only to the low-standoff MOSFET with no other component failures. With different test vehicles, the company was initiating an attempt to mimic the electrical failure of a no-clean paste for low-standoff components, with the intent to design an automotive-grade, no-clean, halogen-free solder paste for enhanced electrical reliability.
DESIGN OF AUTOMOTIVE-GRADE NO-CLEAN HALOGEN-FREE SOLDER PASTE
No-clean flux characteristically consists of a mix of solvents, activators, resins, and rheological additives. During reflow, with normal volatilisation, the activator-solvent system in “dry” flux residue is incapable of undergoing a corrosion reaction, when encapsulated by solid rosin in the presence of moisture, bias, and temperature. However, with lower standoffs preventing proper venting of flux volatiles, activator-solvent system in the “wet” flux residue is available for corrosion, especially with moisture, bias, and temperature. The challenge is to design a flux system with the right rosin-activator-solvent complex that passes electrical reliability, regardless of whether the residue is “wet” or “dry.”
SIR TESTING WITH AND WITHOUT GLASS SLIDE
A standard B24 SIR coupon was chosen for testing purposes. Five no-clean solder pastes consisting of four control pastes and the automotive-grade designed paste were tested as per J-STD-004B IPC SIR conditions. To evaluate the electrical performance of the no-clean pastes for low-standoffs, a glass slide was placed on top of the printed flux deposits on one half of the B24 SIR coupon. Flux deposits, as opposed to paste deposits, were tested to simulate the worst-case low-standoff scenario. B24 SIR coupon was applied with and without the glass slide, in order to mimic the low-standoff condition. The solder pastes included a combination of halide and halogen-free fluxes, T4 powder, and SAC305 alloy. A standard lead (Pb)-free air reflow profile was used in the process.
For the standard SIR testing with no glass slide on top, all of the no-clean fluxes passed the minimum SIR requirement of 100 MΩ. For the test with the glass slide on top, the four control no-clean pastes failed with only the automotive-grade no-clean flux passing. The SIR performance for the five no-clean solder pastes tested on a B24 SIR coupon with and without the glass slide on top. The dendrites for the control pastes were documented with glass slides on top of the B24 SIR coupons.
AUTOMOTIVE-GRADE NO-CLEAN PASTE FOR ENHANCED SIR CONDITIONS
Now that a correlation had been established between low-standoff and an appropriately designed no-clean flux system, the automotive-grade no-clean solder paste was subject to SIR testing for enhanced conditions, as per automotive requirements using a B52 coupon; 10 V and 50 V; 1,000 hr; 0.2mm space width; 93 % RH; 400° C; and a minimum SIR threshold of 5,000 MΩ. The specially designed automotive-grade paste passed this stringent SIR test as well, using the same standard reflow profile as before.
With increased electrification of automotive platforms, the complexities imposed by these systems –higher voltages, higher power components, longer hours of operation, and higher temperatures – on no-clean halogen-free solder pastes used to assemble PCBAs are only going to increase. In the past, the criteria that a no-clean paste had to achieve included standard SIR testing, good printing with response-to-pause, good soldering in air reflow, good coalescence, and the ability to address head-in-pillow defects.
Now, in addition to these requirements, ensuring the electrical reliability of the no-clean solder paste for low-standoff high-power components for enhanced SIR conditions has become equally, if not more, important. It is critical, therefore, to specifically design an automotive-grade, no-clean, halogen-free flux system with the right rosin-activator-solvent complex, that’s electrically safe with no ionic dendritic growth, especially for low-standoff high-power components. In parallel, the no-clean halogen-free flux system should be engineered with the required activity to form reliable solder joints in air reflow conditions.
KARTHIK VIJAY is Technical Manager in Europe, Africa, and the Middle East with Indium Corporation based in the UK.