THE RoHS LEAD-FREE MANDATE

Soldering and its Impact on PCB Manufacturing with Case Studies & Thermal Profiles

[TECH 1] [TECH 2] [TECH 3] [TECH 4] [TECH 5] [TECH 6]

Copyright © 2008 - 2010, 2011 & 2012 by David Jacks

A Tighter Soldering Window? Yes, soldering PCB's will require manufacturers to use solder alloys with higher liquidus temperatures, even as more and more PCB components are becoming smaller and thereby more temperature sensitive like chip scale packages and ceramic capacitors and glass diodes. And to make it even worse, more substrates are packed with greater density of chips and onto higher and higher copper content substrates loaded with heavy ground planes creating more heat sinking which hinders reflow.

Won’t This Make Solder Reflow More Difficult? Yes, the “window of opportunity” between how much temperature a PCBA can handle on the “low end” and the amount of temperature required to solder with lead-free alloys at the “high end” just got a lot tighter. Will it make it more difficult?

No and yes. No, it will not be more difficult if you are willing to briefly preheat your PCBA before attempting solder reflow. Yes, it will be much more difficult if you choose not to preheat your PCB

   The Zephyrtronics AirBath Preheat Makes the RoHS Lead-Free Soldering Challenge a Breeze, Enhancing the High Quality of Solder Joints.

Six Key Thermal Parameters For Processing PCB's at Reflow

Regarding that "window of reflow opportunity," there are six critical parameters that impact and limit any reflow temperature profile when it comes to processing PCB assemblies:

 1.) The substrate’s glass transition temperature.
 2.) The plateau temperature where flux will activate;
 3.) The max temp ramp rate that a chip can handle during heat-up.
 4.) The amount of heat or thermal energy needed to bring a PCB
       up to where solder reflows between its pads and chip leads.
 5.) The maximum temperature and duration at that point that the
       die/chip itself can experience without being damaged;
 6.) The actual reflow temperature of the solder alloy itself, that is
       where the alloy transitions from solid to liquidus.

Six Limiting PCB Solder Reflow Parameters:

1.) The glass transition stage for most FR PC substrates is typically around 160°C to 175°C. Above these temperatures, a substrate becomes prone to unwanted warping, measling, discoloring and delamination. Therefore, the substrate can only be subjected to temperatures above its glass transition stage for a very limited, that is, short periods of time.

Importantly, the ever-popular “flex circuits” have even far lower transition stages and are even more prone to thermal damage. And BGA and CSP packages also have low tolerances to high temps. Preheating the PCB assembly at 150°C with a Zephyrtronics AirBath will be safely below the glass transitions stage of most all PCB’s where not warping, measling, discoloration or delamination can ever occur. And this is a good thing...and why Zephyrtronics pushed the concept so forcefully in the early 1990's.

2.) Flux activation temps used within most soldering processes have range between 120° to 135°C. It is important that flux activation have its brief “moment” in order to clean away impurities, oxides, dirt, oils, surface films that impede and/or prevent good solder wetting and hence quality solder joints. Preheating the PCB at 150° with a Zephyrtronics AirBath will activate your flux and helping prep the pad/lead interface for high quality solder joining.

3.) The industry recommended temperature ramp rate for PCB assemblies is between 2°C and 4°C. The trend today is toward miniaturization of chips such as chip scale packages which makes them very temperature sensitive. Indeed, most all SMD ceramic capacitors and glass diodes can not be heated faster than 2°C to 4°C or they will crack or experience microscopic “fissuring”. The Zephyrtronics AirBaths all have built-in temperature ramp rates between 2°C and 4° to prevent thermal shocking delicate components and chips.

4.)  The energy required to heat up a populated/assembled PCB in order to bring it up to a temperature where solder reflow is possible is dependent upon various factors. These factors include: the material of the substrate, the footprint and thickness of the substrate, the component density, the copper content and/or the amount of grounding planes, and finally the number and weight of heat sinking devices on the PCB assembly. The “heavier” the board assembly, the more energy is required to achieve successful solder reflow. Preheating your PCB with a Zephyrtronics AirBath at 150°C generously supplies the “extra energy” needed and  “stores thermal energy” right in the board itself helping overcoming the heat-sinking characteristics described above.

5.) The maximum temperature that an SMD die/chip can tolerate during reflow without damage is never greater than 260°C! Indeed, there are many chips with even lower thresholds. Generally 260°C (500°F) is the maximum permitted by semiconductor manufacturers. The late Dr. Charles Hutchins -- who founded the prestigious Surface Mount Technology Association -- wrote that IC's exposed to more than 260°C for even five seconds can be irreparably damaged. Because classic tin/lead solders are liquidus around 183°C, they still left ample room in the “process thermal window” for success.

Even so, a PCB assembly that is preheated will achieve final solder reflow at far lower temps than without preheat. Example: The very same solder joint that requires a 370°C (700°F) soldering iron can be made at only 226°C (420°F) if the assembly is briefly preheated for just seconds. That is a delta temp difference of a whopping 144°C (280°F)!

6.) With lead-free alloy, the actual reflow temps where solid state becomes liquidus are now higher than with the old traditional 63Sn/37Pb alloy. Whereas the traditional 63Sn/37Pb alloy had its liquidus at 183°C, most all of the leading Lead-Free candidates that are replacing it have significantly higher reflow liquidus temperatures typically between 220°C and 235°C.

What is eyebrow raising to most engineers, electronic technicians and quality personnel is that there is now little room for “play” between the limiting maximum temperature threshold for chips at 260°C and the liquidus ranges of these new Lead-Free alloys. However, as described above, by preheating your PCB with a Zephyrtronics AirBath at only 150°C for just seconds makes the nearly impossible to achieve with No-Lead solders a breeze.See the study below.

Goal: Measure and record differences in thermal profiles when soldering thru-hole devices on a plated-through, FR substrate PCB assembly in four distinct tests.

Test 1: Generate a quality solder joint using 63Sn37Pb (traditional) solder alloy after a brief preheat (soak) of the PCB at 150°C. Measure the minimum temperature required to achieve solder reflow for a quality solder joint.

Test 2: As with Test #1, generate a quality solder joint using 63Sn37Pb (traditional) solder alloy without the assistance of any preheating of the PCB. Measure the minimum temperature required to achieve solder reflow and to create the solder joint.

FIGURE 1