SMTA International Conference Proceedings

SMT Profiling: Does it Represent Product Reflow Exposure?

Authors: Mitch Ferrill, Matt Kelly, Alec Chen, Sandy Mo
Company: IBM Corporation, Wistron Corporation
Date Published: 9/17/2017   Conference: SMTA International

Abstract: Thermal profiling of Printed Circuit Board Assemblies (PCBAs) is an integral part of any electronics assembly manufacturer’s process development and process monitoring activities. Data is collected to assure that all parameters, which include things such as solder joint temperatures, time above liquidus (TAL), soak times, and peak component body temperatures meet customer requirements, industry standard requirements (IPC, JEDEC, MIL-STD), and component manufacturer requirements. Solder reflow processes that generally require thermal profiling include SMT reflow, vapor phase reflow, wave/selective solder, hot gas rework, and solder pot/fountain. The paper focuses on SMT reflow oven thermal profiling.

A common method of establishing a SMT profile is to attach thermocouples to a product card and pass this card through the reflow oven. The thermocouples are generally drilled and imbedded into solder joints, glued to component bodies, and glued to the printed circuit board (PCB) surface. The thermocouples are plugged into a data collection device that travels through the reflow oven along with the product, acquiring real time temperature measurements. The Process Engineering team passes the thermocoupled product card through the reflow oven, evaluates the output, makes oven setting adjustments, and runs the card through the oven as many times as necessary to achieve the desired temperature and time parameters.

When performing the SMT profile activity, a process engineer passes the thermocoupled card through an empty oven, and the acceptable profile is established based on the data from this empty oven. However, during production, the reflow oven is almost always loaded with multiple PCBAs and rarely, if ever, is a single PCBA passed through the oven with no other loading. It is therefore critical to understand if oven loading adversely affects the resultant thermal profile, as large mass card assemblies can potentially decrease oven zone temperatures. Because many thermal profiles marginally meet all requirements, a slight shift in thermal profile during volume production, can create an out of control condition (not meeting specification requirements). It is important to take into account that production oven loading varies. Profiling optimization work should establish minimum and maximum loadings permissible to assure all parameters remain within specification.

Other factors that are rarely taken into account include: 1) data acquisition tool position with respect to thermocoupled card – leading, trailing, 2) number of data acquisition tools, 3) distance between the thermocoupled board and the data acquisition tools. All of these factors can create a difference in the resultant profile between the profile card and actual product. This paper evaluates the oven loading effects and data acquisition setup during engineering processes definition to ensure process variability is minimized during actual production environments.

Key Words: 

SMT reflow, thermal profiling, oven loading, profile repeatability

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