Get To Know Critical Aspects of Thermal Profiles
There are still people in our business believing that the same thermal profile is adequate and can be used for each and every PCB assembly project. But again, not all thermal profiles are created equal. Each thermal profile is designed specifically for a single PCB project.
But there’s still more to this story. What is taking a backseat against that backdrop are the critical aspects involved with a thermal profile. What it comes down to is thermal profiling is a balancing or juggling act today due to multiple conflicting characteristics and data points, as well as manufacturers’ point of views that the process engineer has to contend with to develop the perfect recipe.
For example, you’ve got the solder paste manufacturer that carefully controls the ramp and pre-heat cycle. He also wants the TAL or temperature above liquidus as high as possible. But then, you’ve got the component manufacturer that wants as low as possible a peak temperature TAL to prevent component damage. We’re not done yet. Here comes the oven manufacturer into this mix. He wants to make sure his oven can make a repeatable and quality profile time after time, and his contributions in some instances may run against the grain of the other two guys.
So, the process engineer creating the thermal profile is caught right in the middle, meaning between a rock and a hard place. He or she is definitely forced to perform a juggling act by having to make intelligent trade-offs if necessary.
Click on this link to our SMT Magazine ‘Zulki’s PCB Nuggets’ column for a full explanation of this crucial topic.
Here are some major tips and hints on what to know about today’s thermal profile for your next PCB assembly project:
- Clearly understand there are at least six critical aspects dealing with creating an effective thermal profile. Get the details in Zulkiís SMT Magazine column above.
- Make sure your EMS provider keeps a disciplined record of their reflow oven maintenance.
- Also, check out to see if the reflow oven has 12 versus six zones. Always better to have 12.
- Understand that your thermal profile must be properly aligned to the mixture of component materials, i. e. plastic, ceramic, glass, etc.
- Factor in a temperature cushion to avoid damaging components and the PCB. Fig. 1 shows a thermal profiler with thermocouple wires to monitor temperatures and record the profile.
- But if the reflow oven isnít maintaining TAL, sufficient wetting and soaking is reduced and open joints and intermittent connections result.
The real savvy process engineers are well equipped to deal with this rapidly evolving assembly step simply because they’ve successfully handled changes relating to creating a thermal profile over the last 10 years. But when you get an inexperienced process engineer into this situation, itís a different story.
They’re not quite ready to create the thermal recipe that’s quality and repeatable and doesn’t have defects. Inexperience brings about such common defects as tombstoning, delineations, or tilting of components, plus at times thereís not enough heat, resulting in intermittent connections or head-on-pillow for BGA components, for example. Plus, CSP and LGA devices have their own issues.