Why can’t I use FR4 with Lead Free Soldering.
You can build a printed circuit board and solder it through a lead free soldering temperatures. It is not advisable! I know some designers that use standard FR4 for lead free soldering applications. The designs are simple single and double sided boards. The companies that they work for have done the due diligence to prove out their design and manufacturing process. On moderate to complex designs they specify lead free soldering compliant laminates in place of FR4. The compliant laminates are much more expensive than standard FR4. They made the upfront investment system by doing the testing and evaluating to save money down the road. They know what they can get away with and have the test data to prove it. However, the results are specific to their designs and their assembly process. When in doubt, play it safe and specify lead free soldering compliant laminate. In order to justify the cost of the more expensive laminate you need to understand what happens to the material at high temperatures.
The main challenge faced by a laminate system is how to survive the lead-free assembly temperatures. The IPC has formally endorsed SAC305 as the lead free solder of choice. Another common lead free solder is SN100C. Eutectic tin/lead solder has a melting point of 183 °C. SAC305 has a melting point of 217 °C. SN100C has a melting point of 227 °C. In order to form a reliable solder joint with a SAC alloy, IPC-7095A “Design and Assembly Process Implementation for BGAs” paragraph 8.4.5 places peak assembly temperatures at 235-245 °C for 60 to 90 seconds depending on the profile. Lead-free assembly temperatures are expected to be as high as 260 °C. Some designs have required assembly oven profiles that rise as high as 270 °C. Regular material cannot withstand the heat for that long or the plated through hole barrels will fail due the z-axis strain. How the base laminate material performs and how long it can survive these higher temperatures are critical. Critical material characteristics and terms to be aware of are as follows…
- Maximum Continuous Operating Temperature (MOT)
- Decomposition Temperature (Td)
- Glass Transition Temperature (Tg)
- Coefficient of Thermal Expansion (CTE)
- T260/T288 Time to Delamination
- CAF (Conductive Anodonic Filament)
What is the Maximum Continuous Operating Temperature? The MOT is a temperature rating issued by Underwriters Laboratories (UL). It represents the maximum temperature that a printed circuit board can operate at without being damaged from heat. The value is generated from manufactured samples provided to UL for destructive testing. Below the MOT the pcb receives no damage from operating temperatures. Above the MOT the pcb starts receiving damage from the heat applied. Epoxy and oxide chemical bonds slowly start to break down or decompose. The greater the temperature the faster the rate of decomposition of these chemical bonds. Every time you solder a pcb at soldering temperatures you damage the pcb from the heat. Lead free solder temperatures are much higher than eutectic tin lead solder. The higher the temperature applied the less time it takes to damage the pcb beyond the point where it shall function reliably.
What is the Decomposition Temperature? By definition this is the temperature at which a 5% weight loss occurs by thermal gravimetric analysis (TGA). The decomposition is the breaking of chemical bonds in the resin system. The resin in the laminate is basically burning up. A 2% to 3% weight loss of this nature will adversely affect circuit performance. The damage caused from decomposition is cumulative. This places a limit on the number of lead-free assembly cycles at temperature on a printed circuit board.
What is the Glass Transition Temperature? By definition this is the temperature at which a polymer changes from hard and brittle to soft and pliable. Despite its name, it has nothing to do with the fiberglass reinforcement material in the printed circuit board. This value refers to the resin system when it changes from a hard glassy state to a soft rubbery state. It is also the critical point in thermal expansion. Below the Tg value the rate of thermal expansion is low. Above the Tg value the rate of thermal expansion is higher by
a factor of up to 10. The Tg has historically been the designer’s gauge for reliability.
What is the Coefficient of Thermal Expansion? By definition this is a material’s fractional change in length for a given unit change of temperature. The common unit of measurement is ppm/°C, parts per million per degree centigrade. 1 ppm is equivalent to 0.0001% of total observed dimension. A material rated at 250 ppm/°C would change 0.025% in dimension for every degree change in temperature. On a .100” thick board over a 100°C temperature range there would be a total thickness change of 2.5% which equates to 0.0025”. Glass, copper, nickel and gold all have fixed expansion rates up to their melting points. The circuit board resin system expands at one rate below the Tg and another rate above the Tg. This is why the Tg value is important. High Tg materials tend to be more stable. Low Tg materials tend to be less stable since they have a wider range of high expansion once you pass the Tg value. Copper has a Coefficient of Thermal Expansion of 17 ppm/°C. A printed circuit board made on FR4 (Isola/Polyclad FR226) material has a Coefficient of Thermal Expansion of 50 ppm/°C below Tg and 250 ppm/°C above Tg. On a lead-free process the difference in expansion on this type of laminate creates tremendous strain on the hole wall. At lead-free temperatures the laminate will expand by approximately .004” possibly resulting in lifted lands and hole wall
What is the T260/T288 Time to Delamination? This is a test defined by the IPC under IPC-TM-650 Test Method 18.104.22.168. A test sample is incrementally raised in temperature 10°C/min to 260°C (or 288°C) and then dwells at 260°C (or 288°C). When an event occurs such as delamination, cracking, moisture release, stress relaxation, decomposition or a sudden movement a sensor detects the change. The time in minutes from the start of the dwell time to the event at 260°C (or 288°C) is the time to delamination. The Time to delamination has been observed to decrease as the board gets thicker. The maximum expected lead-free
assembly temperature is 260°C. The T260/T288 test provides an indication as to how much time a printed circuit board may withstand the higher temperature.
What is CAF? CAF is an acronym for Conductive Anodonic Filament growth. Bell Labs first identified CAF in 1976. Field failures were later identified in 1980. CAF is formed as a result of several events. As an epoxy resin system is heated there is the possibility that the resin shall separate from the fiberglass reinforcement. The separation forms an open channel at the resin to fiberglass interface along the length of the fiberglass strand. This separation is random and next to impossible to find by X-section. When an assembled board is exposed to high humidity the CAF channel fills with water. The copper corrodes and an electrochemical pathway develops. The water acts as an electrolyte, the copper circuitry becomes the anode and cathode and the operating voltages act as the driving potential. Basically a filament containing copper grows along the channel formed at the epoxy resin and glass interface. The growing filament ultimately forms a short between two unlike conductors resulting in a field failure. CAF is the hidden danger that lurks behind lead-free processing. Where as other assembly related defects are easily observable, CAF is not. CAF shall not appear in a printed circuit board until after several months of active use within certain environments. Laminate suppliers advertise lead-free materials as being CAF resistant. This is not a guarantee that
lead-free advertised laminate shall be CAF free. However, the ability of the lead-free advertised material to resist CAF formation is far better than standard FR4.
It should be noted that the Td, Tg, CTE and T260/T288 values are derived from uniform samples. The thermal dynamics of a design have not been taken into account. How the circuit is laid out will greatly affect how the board survives the assembly process. Lead-free assembly is not a drop in process. The board design, assembly practices and raw materials must be evaluated. Even if the board survives the assembly
process, CAF failures may occur in the field. Keeping all these factors in mind it is advisable to select more thermally robust and higher priced laminate compliant with lead free soldering in place of standard FR4.