Issue link: https://iconnect007.uberflip.com/i/1035071
OCTOBER 2018 I SMT007 MAGAZINE 75 The soak profile had zero BGA joints with more than 8% by area, voiding, whereas over 10% of the straight ramp profile results had voids between 12% and 16% of the area. The result of adding a preheat soak and a slower ramp rate to the peak temperature significantly reduced the extent of voiding in BGA compo- nents when compared to a fast ramp to peak temperature. Extending Time Above Liquidus Another adjustment to the reflow profile that has proven to reduce voiding is extending the time above liquidus. This is especially effective with large surface area components like MLFs, DPAKs, TO-252 and other bottom terminated components (BTCs). In order to verify the effect of the impact of extending the time above liquidus the concept was captured on video using a reflow simula- tion. Test Method A solder paste deposit was placed on a large metalized surface. A piece of glass was placed over the deposit. The video camera captured the image of the solder paste as it went through a reflow profile. Images were captured at key temperatures of the reflow profile to explain the formation and elimination of voids. RESULTS AND DISCUSSION Results The formation of gas bubbles from the evap- oration/boiling of organic solvents can read- ily be observed. This outgassing before the alloy reaches liquidus is an explanation for the reduced levels of voiding seen when a soak preheat profile replaces a straight ramp preheat. A greater volume of the gas produc- ing solvents are driven off before the solder spheres melt and coalesce in a liquid phase; see Figure 6. Figure 5: Voiding results. Figure 6: Outgassing during a straight ramp reflow profile at 155°C.