THE RoHS LEAD-FREE MANDATE

How Are Lead-Free Solder Alloys Different From the Traditional 63Sn/37Pb Solder?
While there does exist many lead-free solder alloys, the greatest focus has been on finding a “drop-in” replacement for the ever-popular “eutectic” 63Sn/37Pb or 60Sn/40Pb solder alloys that have been common staples within the electronic industry for over 50 years. Unfortunately, nearly all the leading candidates have higher melting points which impact everything from high-volume production reflow ovens to lower volume bench-top soldering and desoldering.

Good Grief! Won’t This Make Solder Reflow More Difficult?
Let’s see if we are getting this straight?: We now are going to require manufacturers to use solder alloys that have higher reflow temperatures, at the same time, more and more PCB components are becoming smaller and thereby more temperature sensitive such as tiny CSP’s, 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.

So, won’t this new requirement of having to use a higher melt-point solder make achieving reflow more difficult? Certainly the “window of opportunity” between how much temperature a PCBA can handle on the “low end”, and the amount of temperature required to achieve solder reflow with lead-free alloys at the “high end” just got a lot tighter. But will it make it more difficult? No and yes. No, it will not be anymore difficult if you are willing to briefly preheat your PCBA before attempting solder reflow. However, it will be much more difficult if you choose not to preheat your PCB assembly before attempting reflow.

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 temperature 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 can reflow between its pads and the leads of the chips.
 5.) The max temperature and length of time at that temperature that the
       die/chip itself can experience during reflow without being damaged; and
 6.) The actual reflow temperature of the solder alloy itself, that is where it
       transitions from  solid to liquidus.

Examining These Six Limiting PCB Reflow Parameters:
1.) The glass transition stage for most FR PC substrates is typically around 160°C to 175°C. Above these temperatures, the substrate becomes prone to unwanted warpage, measling, and delamination. Therefore, the substrate can only be subjected to temperatures above its glass transition for limited, that is short periods of time. It should be noted that the ever-popular “flex circuits” have even lower transition stages and are even more prone to thermal damage. 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.

2.) Flux activation temperatures 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 assembly at 150° with a Zephyrtronics AirBath will activate your flux and thereby help 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 and length of time at that temperature that the die/chip within the component itself can experience during reflow without being damaged is never greater than 260°C! Indeed, there exist some components with even lower thresholds. Generally 260°C (500°F) is the maximum permitted by the manufacturers of most all semiconductors. The late Dr. Charles Hutchins who founded the prestigious Surface Mount Technology Association wrote that a semiconductor that sees over 260°C for even five seconds is irreparably damaged. Since the popular 63Sn / 37Pb solder alloy had a liquidus temperature of 183°C, this left substantial room in the “window” of a typical solder reflow process profile for success. By preheating your PCB assembly with a Zephyrtronics AirBath you can achieve far lower reflow temperatures than you can without it. 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 first briefly preheated for just seconds with a Zephyrtronics AirBath. That is a delta temperature difference of a whopping 144°C (280°F)!

6.)  The actual reflow temperature of the solder alloy itself where it goes from solid to liquidus will now be higher with the new Lead-Free solders 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 now a breeze.

Goal: Measure and record the differences in thermal profiles when soldering through-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 and to create the 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.

Test 3: Generate a quality solder joint using a Lead-Free (96.5Sn/3Ag) solder alloy (See ZeroLead® Solder Paste) after a brief preheat (soak) of the PCB at 150°C. Measure the minimum temperature required to achieve solder reflow and to create the solder joint.

Test 4: As with Test #3, generate a quality solder joint using a Lead-Free (96.5Sn/3Ag) 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.

Result and Observations: Achieving successful solder reflow with lead-free solder required higher temperature applications than those of traditional 63Sn/37Pb solder. Indeed, soldering Lead-Free without any preheating of the PCB assembly required the application of approximately 100°C more temperature than when preheating was included into the controlled test sample. (See Figure 1).

Perhaps, most notable and most encouraging was that soldering through-hole components with a plain soldering iron could be done at lower temperatures with Lead-Free solder than with traditional leaded-solder IF a preheating soak at only 150° was made prior to the attempt to reflow. That’s a headline in and of it self: You can solder with Lead-Free solders at lower temperatures, if you preheat first, than you can with traditional 63Sn/Pb solders