This paper presents the results of an initial evaluation study to determine if a printed circuit board’s (PCB’s) profile can be verified by using only 3 thermocouples (T/Cs). Herein, this paper describes two processes: a “characterization” process and a “verification” process.
It is important to mention that the techniques herein are not suggested to replace a thorough T/C procedure for initial product/process development and qualification. Instead, these techniques are geared for use in process monitoring once a process has been qualified. Also, while only one type of reflow oven is used in this experimentation, these techniques can be applicable to a wide variety of reflow ovens.
In the characterization process, a T/C is applied to a lowmass component (L), an intermediate/sensitive component (S), and a high-mass component (H). In the verification process, the 3 T/Cs (L1-L3) are placed on the laminate, along the top, leading-edge surface of the PCB.
This study uses a desktop computer’s motherboard as the Test Vehicle (TV). A 20-channel temperature recorder (a.k.a., “M.O.L.E. ® Temperature Recorder”) is used to attach 20 T/Cs to the TV – 3 attached along the top, leading-edge and the rest attached at various locations and components (sensitive/critical and non-critical) around the TV. A baseline, Pb-free profile is identified as the desirable profile and two response variables are studied: peak temperature and time above liquidus.
By simulating changes to the oven – performed by altering belt speeds and a temperature zone from the baseline profile – evidence shows that by putting tight specifications on the two response variables for the temperature profiles of the 3 T/Cs located on the surface of the laminate’s leading-edge, one can effectively verify that the board’s characterization profiles are still within specification.
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