Honey, I Shrunk the Process Window
BY MARYBETH ALLEN
With the electronics industry making its move to lead-free manufacturing, attention is focused on solder. Melting temperatures for reflow and wave applications of lead-free solders - primarily SnAgCu (SAC) alloys - have increased. Higher melting temperatures require higher peak temperature ranges, and have caused the focus to shift to the entire product. Now, we profile to achieve specifications of the solder paste plus the components and substrate.
The challenge is achieving a higher melting temperature for the solder, while remaining within the specifications of components and the substrate. Finding a reflow oven and wave solder machine recipe for solder requirements is similar to the process used in the past; profile and optimize. However, this must now be accomplished without exceeding maximum component and board temperatures, causing a dramatic change from the way we profile and optimize these products.
Many commonly used SAC alloys melt at 217°C, with peak temperature ranges up to 265°C. This infringes on maximum limitations of many sensitive ICs, crystals, power amplifiers, and more. Many of these parts have a peak temperature tolerance of 235° to 245°C. Since peak temperatures seen with typical SnPb or SnPbAg solders was significantly lower - 205° to 230°C - the risk to these parts was not frequently a factor. Many improvements have been made over these last couple of years to develop components that have higher tolerances, but many still remain at 235° to 245°C. Tighter limitations of lead-free solder paste specifications, coupled with the concern to avoid damaging components and substrates, had caused the process window to become much smaller.
Temperature inside the component is also a factor. Manufacturers will look at peak temperatures and rising and falling ramp rates to assure there is no damage to the IC, such as cracking or lifting of the wire bond, or cracking of the die. This is done by inserting a thermocouple (TC) through the package directly to the wire bond or die. It is not unusual to also look at the temperature at several locations on top of the component to determine the delta T across this part. The industry is also looking closely at the profile of ball grid arrays (BGAs). Voiding, proper ball collapse and shape, and intermetallic structures are carefully scrutinized and controlled by proper profiles. TCs are often put under the BGA right at a ball.
Most substrates have not changed to specifications with higher glass transition temperatures. Depending on the board layout, thickness, and metal layers, PCBs may be at risk for charring, delamination, or simply discoloration.
To ensure an in-spec process for the total product, additional TCs are used to gauge the results at more varied component (small, medium, large) solder joints, inside components, under BGAs, and on bare areas of the substrate. This means that process windows of these individual specifications are not always the same. An example of this would be the TC attached to a lead on a QFP that may include temperatures and times for maximum rising slope, time above liquidus, peak temperature, and soak time between some established temperature points. However, the process window relevant to the TC embedded in the die may be maximum rising slope at a different rate, a lower peak temperature, and a specification for maximum falling slope. There would be different process windows for TCs attached to the substrate and other components. Therefore, results must be evaluated differently for each of the TCs according to their individual process windows. This task may lead to more time spent on developing optimized recipes. Electronics companies of all sizes are looking for profiling equipment selection that will take all these factors into consideration and use powerful software to automate this process; reducing the amount of time needed to find an optimized solution.
Many of us remember changes we dreaded but successfully accomplished with the advent of fine-pitch and no-clean solders. To ensure good quality and reliable products, we are fine-tuning our thermal process. Once again, we are challenged, but with the right thermal tools and processes, lead-free manufacturing will shortly become a standard technology in the assembly of device packages and PCBs, and all the trauma will be a mere memory.
MARYBETH ALLEN, general manager, may be contacted at KIC, 15950 Bernardo Center Dr., # E, San Diego, CA 92127; 858/673 6050; E-mail: firstname.lastname@example.org.