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tech.com
July, 2016
Top-Side Preheat and Avoiding Board Warp in Selective Soldering
By Jonathan Wol, President, Pillarhouse USA, Inc., and Simon Smith, Marketing Manager, Pillarhouse International Ltd. C
losed-loop top-side preheat generally refers to any preheating unit that presents energy from above the PCB and is used for heating
the board to a specific temperature, as well as holding it at this value throughout the process. The system must include some form of tempera- ture measuring and feedback system through which a real-time control loop can be established. The heating method can be convection, infrared (low-to-medium wavelength) or quartz elements. In selective soldering, the predominant heat-
ing method is infrared (IR). IR can add more ener- gy to raise the temperature quickly at the begin- ning of the cycle and is able to actively control the heat as required in real-time during the process. Convection adds the same benefit of holding the board at temperature once the target temperature is achieved but the slow rate of temperature change would increase the cycle time at the initial ramp of the process. The value of the closed-loop top-side heater is
twofold: It provides the primary energy to reach the target temperature, and just as critically, it ensures that the temperature remains constant throughout the process. The closed-loop system keeps the process homogeneous throughout the
Depending on the PCB
thermal characteristics, the board can lose upwards of 20°C before soldering begins.
cycle. It also eliminates the variables associated with changes in incoming board temperature and
board thermal requirements as a direct result of the soldering process. The challenges of using non-closed-loop top-
side preheat-equipped machines have been well- documented as early systems were gantry-based
A typical scenario for a non-closed-loop top-
side equipped machine is that a PCB with several of the same components (assume connectors) would see different requirements in dwell times or travel speeds as the machine progressed from the first to the last connector. These settings are pro- grammable, but the difference becomes more pro- nounced when the incoming board temperature into the system is considered. This means that the variable programming method is not consistent for all foreseeable conditions. The reason for the required process change is
Top-side preheat unit.
units that made top-side preheat difficult. While bottom-side heaters prior to the soldering process are capable of achieving soldering temperature, the inability to continue to add heat as needed dur- ing soldering results in a less stable process and increased variability. In some cases, depending on the PCB thermal characteristics, the PCB can lose upwards of 20°C before soldering begins. Once the PCB is removed from the preheater,
the temperature begins to drop immediately. Heat is added locally during the soldering process, but the effect of the localized preheat is product-specif- ic and cannot be assumed to be the same for all PCBs. The rate of temperature decline is directly related to the board design and must be tested to get accurate results.
the energy added during the actual soldering changes the thermal requirements for the subse- quent processes. With closed-loop top-side heat, the process for each connector is the same, making it possible to use the same template or part pack- age to cut and paste the entire product. The result is a significant reduction in programming time, program complexity and much greater process uni- formity.
Board Deformation Board warping is the unintended change in
the geometry of a printed circuit board (PCB). Board warpage is the general term used to describe an altered PCB shape, regardless of the shape itself (bow, twist, etc.). One of the primary challenges of processing a
warped board is that the transition may take place as part of the soldering cycle. Therefore, the prior shape may not directly correlate to the shape of the PCB at the time of soldering. One common source of board warp is unbal-
anced PCB layers and misaligned fibers. To mini- Continued on page 60
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