The higher soldering temperatures and smaller process-windows in reflow soldering of lead-free PCBA’s means higher demands. To make perfect SMT reflow oven profile Board establish an accurate method for temperature profiling and a control became more important.
In this article I will explain the how can make a profile board to measure and develop a solid reflow profile to meet quality requirements.
Selection of Measuring Points on SMT Reflow Oven Profile Board
Measuring points shall be selected so that the coldest and the warmest solder Joints are measured and so that the specified maximum package top Temperatures are not exceeded. When selecting measurement points, the Following criterion shall be considered:
- Large ground plane area
- Component size (small and large mass)
- PBA population (mass distribution)
- Component material
- Temperature sensitive components
- Component color (applies to IR)
Methods to jointing Measuring probes
We wanted to know what method to use when attaching probes to the board and components. There are 3 major methods today
- High Temperature solder.
- Tape (Polyimide tape and aluminum tape).
According to a report from National Physical Laboratory ISSN 1473 2734, the best method is to use high temperature solder, adhesive is almost as good as high temperature solder. Polyimide tape is not recommended. Aluminum tape is also a good alternative. Aluminum tapes show a slightly higher temperature than adhesive and high temperature solder.
How to Attach thermocouple to the Component terminal
The probes can be attached using adhesive or high temperature solder. The quantity of adhesive or solder shall be as small as possible in order not to affect measurement result. The probe should be located close to the lead. During attachment the thermo element wires shall not be twisted, and joined as close to the thermo elements isolation as possible.
Soldering Wire Recommendations for thermocouple attachment
The probe is soldered using high lead content, high temperature (~300°C melting point) solder (e.g. Pb/5Sn/1,5Ag melting point 301°C or Pb/1,5Ag/1Sn melting point 309°C) can be used.
Attaching a Thermocouple to a BGA Solder Balls
Drill holes through a pad from the PCB’s primary side. Use a drill that has a smaller diameter than the pad. A part of the pad has to be left after drilling so that there is a surface to solder to. Put the thermocouple through the hole from the secondary side. The tip of the thermocouple should be approximately 0.1 mm above the pad surface. Secure the thermocouple with heat resistant
Glue or tape on the secondary side. Screen print, assemble and reflow the board as usual.