What damage does the assembly process do to a pcb? (part 1)
A colleague contacted me the other day with a topic that would make an excellent post on this blog.
“How can we solder boards with a Tg of 180°C or even 200°C at temperatures of 225-245°C without damaging the board? Even with leaded boards the peak reflow temperatures are way above the board’s Tg. How is this possible?”
The answer is simple. Every time a printed circuit board is exposed to soldering temperatures it becomes damaged. This is the case not only for Lead-Free soldering applications but also for eutectic soldering consisting of tin-lead.
Tg is one of several parameters to be aware of. In the case of Tg most designers refer to the value as a gauge of stability. A high Tg product has less thermal expansion in the Z axis. Below Tg the laminate epoxy resin system is hard and solid. Above Tg the epoxy becomes soft and pliable. When the epoxy becomes soft and pliable the rate of expansion can be up to ten times the expansion rate when compared to the expansion rate below the Tg value. Tg 180 material is more stable than Tg 140 laminate due to the 40 degree range of the epoxy system remaining hard and solid.
The parameter that best determines when the board starts to take thermal damage is referred to as the Maximum Continuous Operating Temperature, aka MOT. MOT is a parameter that is tested for and certified by Underwriters Laboratories, aka UL. A bare board manufacturer shall build a qualification sample through their most destructive process and submit it to UL for compliance testing. A board with a certified MOT of 130 deg C shall take no thermal damage in the field when exposed to temperatures at or below 130 deg C. Above the MOT value the epoxy starts to breakdown at the molecular level. As the molecular bonds break due to heat stress, energy is released. The released energy appears as smoke. The higher the temperature that the board is exposed to the faster the damage occurs. Damage is cumulative.
Dry baking moisture out of a pcb is a good example of the damage threshold. The dry bake temperature must be above the boiling point of water (100 deg C) but below the MOT. Theoretically speaking you can dry bake a board with a MOT of 120 deg C at 110 deg C for a year and the board shall take no thermal damage. Likewise if the dry bake temperature is 121 deg C the epoxy would start taking damage. Theoretically speaking at one degree above MOT it would take months to do any real damage. At 300 deg C the board is damaged so quickly that delamination occurs in less than a minute. The higher the temperature the quicker the rate of damage becomes. In reality the MOT is the tested value. The actual board may exceed this by a few degrees.
Every time a printed circuit board is exposed to assembly temperatures the epoxy takes damage. This is the case for conventional eutectic soldering temperatures. This is where the decomposition temperature (Td) value comes in. The higher the Td the slower the rate of burn. Materials with a Td at or below 300 deg C are fine for eutectic tin-lead soldering. Materials with a Td above and beyond 300 deg C are more suited for lead free soldering. In this case, the higher the Td value the more thermally resistant the epoxy is. The higher the better. Highly rated Td material shall still burn away but at a slower rate of decay. Lead-Free assembly compliant laminates are typically phenolic cured epoxy systems. Phenolic cured epoxy systems typically have a higher Td compared to the traditional dicey cured epoxy systems. It is for this reason that dicey cured epoxy systems are not considered Lead-Free assembly friendly.
Soldering has always done damage to printed circuit boards. Tin lead soldering hasn’t been damaging enough to be a big concern at the eutectic soldering temperatures applied. Lead free soldering is and should remain a concern to everyone.