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Paul Austen, ECD, explains why information gathering and reflow profile characterization can be wasted practices without the verification step to maintain proper oven operation. Reflow oven profiles can be out of spec due to flux residue build up, belt speed glitches, and other small changes that deliver significant alterations to the assembly being processed.
Thermal profiling is always a hot topic, and 2009 saw shifts in perception, additional methodologies for implementation, and surprising results about OEM/EMS practices. All served as catalysts for treating profiling as an ongoing process rather than a single step. This is especially important for downsized companies who must employ fewer people, many of whom are filling several roles and in some cases picking up profiling for the first time.
The year began with J-STD-075, dealing with thermal sensitivity of passive components subjected to the higher temperatures of lead-free soldering. Alarmingly, these components pass standard tests and inspections but degrade more rapidly than expected, failing in as little as 6 months. This can result in OEM or EMS liability — especially if it occurs in the high-reliability military or medical sectors that are expected to drive growth in the electronics industry.
J-STD-075 recommends that component manufacturers mark sensitive components as such, for easier accommodation during board assembly. This is a good first step in letting EMS companies know there is a thermal-sensitive component in the assembly. Through a survey conducted at IPC’s APEX tradeshow and on the Internet, it was shown that OEMs often hand over a job to an EMS company specifying lead-free, without further instruction regarding thermal sensitivity. EMS companies, furthermore, often don’t ask about specifics. A need to facilitate communication between the two groups, especially throughout the profiling process, is obvious.
Three-stage Thermal Profiling
To help OEMs and EMS providers understand thermal profiling as an ongoing process, the process can be broken into three distinct stages: the thermal “requirements” for the components, “characterization” of the oven, and “verification” of future production runs. All are vital to the success of board assembly.* No matter what is used as the basis of dialogue, however, EMS providers and OEMs must have the discussion.
Figure 1. Temperature delta maps showing "poor" and "better" oven uniformity. In general, ±2° or 3°C is the expectation of a reflow oven in U.S.-based higher-reliability projects.
The first stage is gathering all the requirements for a good profile, bringing together the reflow limits of the components and the needs of solder paste. In the second stage, the nature of the oven and resulting thermal profile must be characterized to help produce an oven "recipe" to accommodate the requirements of the components and solder. Good thermal profiling software will help in this process, using the characteristics of the oven to help produce a recipe that satisfies all requirements. The third stage, verification, demands repetition of the profiling step either daily or whenever a different recipe is run in the reflow oven, whichever is most frequent. This is done to compare the actual reflow profile to the target profile determined in the characterization stage. This third stage of verification is often forgotten, becoming “the hole” in the process.
Reflow Process Verification
The previously mentioned survey highlights this “hole,” and stresses the need for verification. While over 90% of OEMs and EMS companies said they were diligent about making a target profile (characterization), nearly 60% admitted that they drop the ball when it comes to verification. Aspects of the reflow process can change during production, specifically the oven’s heating performance. No matter how good the oven, nor what the brand, it can change (Figure 1).
Dust and flux residue on fan blades affect convection rate. Your oven may produce correct air temperatures, but those temperatures may not exist at the product level. Recipes can change, by design or by mistake. Speed of the conveyor can be altered, changing the time spent in each zone, and thus affecting the entire thermal profile as the board passes through each zone. It is necessary to verify the oven is still producing the established target profile. This is stage three of the profiling process: verification. And it is this step that is most neglected. Unfortunately, it is the step that tells us how we are doing, and alerts us if there is a significant change in the oven.
Figure 2. New profiling products (Shown: OvenCHECKER from ECD) can verify oven performance using a one-piece pallet instrumented to indicate profile conformance to the original target profile.Why is verification not routine? Downtime for thermocouple (TC) attachment was often cited in the survey; but several studies this year suggest solutions. Binghamton University, Binghamton NY, in conjunction with Unovis Solutions, showed that when done correctly, following a previously run characterization profile, a three-TC board can produce an accurate verification profile.1 Other timesaving profiling products are constantly being introduced (Figure 2).Conclusion
In the light of this year’s developments, it is imperative the OEM and EMS provider communicate regarding thermal sensitivity of components as part of a dialogue that should be second nature. Since military and medical industries will be large contributors to electronics industry growth, it is prudent to begin profiling J-STD-075-marked passive components so inexpensive devices don’t compromise high-rel applications. Finally, the industry must approach thermal profiling as a process with three distinct stages encompassing requirements, characterization and verification. Through complete thermal quality management* of the reflow soldering process, we can eliminate the most common causes of initial and future assembly failure.
* The Thermal Quality Management Program (ThQM™) was designed to walk both OEM and EMS through those three stages, providing sample questions to facilitate discussion.
REFERENCES:1. Austen, Paul, “Oven vs. Board Profiling for the Production Environment,” SMT October 2009, http://www.electroiq.com/index/display/smt-article-display/343296/s-articles/s-smt/s-home-page/s-2008/s-10/s-oven-vs-board-profiling-for-the-production-environment.html.
Paul Austen is a thirty-year veteran senior project engineer with ECD, in Milwaukie, OR, and holds a degree in electrical engineering from the University of Portland. Contact him at email@example.com.
Jack Crawford, IPC, discusses the need for ECA/IPC/JEDEC J-STD-075, “Classification of Non-IC Electronic Components for Assembly Processes.”
Paul Austen, ECD, explains how to characterize thermal behavior of passive components to prevent functional degradation of temperature-sensitive devices during soldering.