Take for example the thickness of a PCB. If it’s a 62 mil thick board, there will be different thermal characteristics associated with the different zones compared to a 93 or 125 mil thick board. The thicker board will take more time to heat up the board’s different segments. Likewise, layer count is important. The stack up structure containing the number of power and ground planes determine the amount of heat the board needs to absorb in any given cycle to create a perfect thermal reflow.
Component types are also an important consideration for generating a perfect thermal profile. If the board is mostly populated with through-hole components and a few surface mount (SM) ones, it will require less heat than a largely SM populated board. Moreover, a full SM component populated board with CSPs, BGAs, and 0201 passive components will have a vastly different thermal profile than even one equally populated with through-hole and SM components. Further, air pressure in a reflow oven must be carefully monitored to avoid blowing away smaller packaged devices such as 0201 and 01005 components.
Aside from these key factors, planning for an accurate thermal profile includes avoiding thermal shock by assuring gradual temperature increases or decreases between different oven zones, carefully monitoring soak time and peak temperatures, and following the specifications provided by the solder paste manufacturer. Solder paste manufacturers provide a specification chart associated with a particular paste.
It provides detailed information about ideal temperature ranges in different oven zones. Included are pre-heat, soak, and re-flow cycles and finally, to cooling down cycles during the final stages of re-flow in the oven. If an EMS company follows these charts carefully, they can nail down the thermal profile perfectly.
If these specifications aren’t followed to the letter, a poorly developed thermal profile can result in the creation of voids and de-wetting if too hot and if too cold, non-wetting and solder joint fractures.