Reflow ovens often show degrees of variability from profile to profile. This may depend on the distribution of components on the board, especially those that are slow heating, heat-sensitive, or of high mass. In general, reflow systems cannot generate one single reflow profile producing capable thermal results for all products.
For instance, a large BGA package on the PCB may not allow more than five degrees of variation near the peak of the reflow profile curve. Therefore, even while the BGA joints show good soldering, there is a probability of frying some other smaller components nearby the BGA package.
Variables during reflow can also be the result of several external factors. There may be limited control for some factors, but others could be uncontrollable. For instance, in some cases, the PCB may be non-uniform, its components may have varying thermal characteristics, or the tolerances of the process controller could be the major contributor. Even the exhaust could contribute as an external factor.
Oven loading is another major factor when creating custom reflow profile for a high-layer-count PCB. The reflow oven characteristics depend on the number of PCBs passing through it, as the total mass of the PCBs and their speed through the oven influences the rate of rise of temperature. Usually, the load capacity of the reflow oven is measured in boards per minute, and this value differs when only a single PCB is passing through as against a batch of several PCBs passing through at a time.
Customers often demand demonstrable settings of the custom reflow profile for their boards. It may be necessary to demonstrate that a given setting fulfills the requirements of a thermal profile for the board, without damaging any other component on it. Sometimes there are requirements to create documentation as evidence that a particular assembly is indeed within specifications. One of the advantages of creating custom profile for a board is it brings a total visibility to the lead-free reflow process when handling that board.
Automated Methods for Lead-Free Reflow
A reflow profiler, such as the one made by KIC, is the most popular method assemblers use for profiling groups of boards they assemble. The instrument works equally well when profiling individual boards automatically and continuously. Assemblers also use it for a cluster of boards consisting of two or three categories.
The KIC profiler has a Navigation prediction software accompanying it. This helps to drive the generated profile deeply within the specifications of the board. Typically, actual profiles need to be run on some boards that match the representative profile for that group. The process must be repeated periodically to ensure the settings remain valid. Used along with the Navigation prediction software, the KIC profiler saves much time and effort when creating lead-free reflow characteristics for high-layer-count PCBs.
Lead-free reflow of high-layer-count PCBs need not be a tiresome exercise provided it is possible to set up a custom reflow profile for a group of PCBs with similar thermal characteristics. Using modern thermal profilers makes the job economical and fast.