Posted on 29 May, 2019 by Richard Barratt
Printed circuit board assemblies (PCBAs) are often viewed as commodity items. But consistently producing them in volume can take time to perfect.
Electronics manufacturing services (EMS) providers may well have their own preferences when it comes to their choice of machine type and brand. But, broadly speaking, the process steps that they go through to produce PCBAs are the same.
There is however one specific step that can make all of the difference when it comes to quality and consistency - creating the surface mount (SMT) reflow oven profile.
The reflow oven is used primarily for the reflow soldering of surface mount electronic components to printed circuit boards (PCB).
If you fail to profile the oven correctly, then all of the hard work and effort that has gone into loading feeders, programming the machines, optimising the build and then running the production line will be wasted.
So what should your EMS partner be doing to perfect their reflow oven profile? Let's find out.
Working on the baseline
The New Product Introduction (NPI) engineer will usually start the process by selecting an oven profile they have used before and that is stored on their system.
Over time, some EMS providers may develop a range of "baseline" profiles, which can then be selected depending on the PCBA they are working on. The time taken to perfect the baseline profile will pay dividends as most assemblies will then fall within one of them.
Baseline profiles may include:
- One dedicated to densely populated assemblies with elevated temperatures
- One dedicated to "standard" assemblies
- One dedicated to assemblies that contain sensitive parts and require lower temperatures
Using their skill, judgement and experience, the NPI engineer will select the profile and temperature settings that they believe are best suited to the new assembly.
Requesting a sample PCBA
Ideally, the OEM will have provided their assembly partner with a populated printed circuit assembly. If one isn't provided, then the NPI engineer may request one for use during the set-up process.
The NPI engineer will attach thermocouples to multiple locations across the sample board. These should be attached to a range of components with varying degrees of heat dissipation - i.e. a transformer, ball grid array (BGA) and passive devices.
Dependent on the size of the PCBA, it is recommended that between three and six thermocouples are attached and it's critical they are connected properly.
Quite often EMS providers will tape these in place but, unfortunately, there is a risk that they will lift during the process, which can result in the air temperature being measured rather than the board and lead temperature.
For a secure connection we recommend applying:
- High temperature solder to secure the thermocouple to the component lead
- An adhesive, such as Chipbonder, ensuring residual glue is not found between mating surfaces
Special attention should be given to BGAs. For the best results, a thermocouple should be placed underneath the device which makes direct contact with the terminations and enabling a small hole to be drilled through the underside of the board. But this can be a destructive process so will require a sample PCBA from the outset.
The PCBA will then be run through the reflow oven with thermocouples attached to a data logger. It's important that that the data logger is positioned correctly so that it doesn't affect the movement of the PCBA through the oven.
Once the PCBA has passed through the oven, the results from the data logger will be downloaded into the chosen oven profiling software. And this is where the first real analysis of the profile begins.
It's all in the detail
Each solder paste manufacturer will recommend a baseline profile with + and - tolerances. Where possible, the EMS provider will try and achieve a result in the middle of these tolerance bands. However, certain component requirements may require them to work at the upper or lower limits of these tolerances.
The NPI engineer should be paying close attention to the following three areas (based on the assumption that a typical lead-free alloy with a melting point of 217 degrees Celsius (°C) is used):
- Ramp - this is the rate of temperature increase measured in °C per second and should be between 1-3°C per second. This will be more noticeable at the beginning of the profile where the board temperature changes from ambient to 120°C. Failing to comply with this could lead to component damage and this is one area component manufacturers typically specify.
- Soak - normally occupies 33-50 per cent of the total heated tunnel length and exposes the PCB to a relatively steady temperature that will allow the components of different mass to be uniform in temperature. The soak zone also allows the flux to concentrate and the volatiles to escape from the paste.
- Reflow - this is the area where the temperature is elevated to between 230 and 250°C. A critical measure is the time above reflow, which typically lasts between 45 and 90 seconds. It's important to check component specifications for maximum temperature.
Fine tuning the profile
Depending on the data logger output, the NPI engineer may decide to "fine tune" the profile further. Using the oven profiling software, the NPI engineer has the ability to amend certain oven parameters (speed/temperature) through the oven profiling software and to then view the "predicted" results.
In our experience, we have found the prediction tool to be highly effective - however, it does rely on ensuring the oven is accurately represented within the software. To do this, the software will need to understand the oven make and model, the number of zones and the length of each zone.
Providing the NPI engineer is happy with the predicted results, they will then make physical changes to the oven parameters and store the profile under a unique reference number. The reference number will typically consist of the item code, the revision level and whether the profile is for the top or bottom side of the PCBA.
So there you have it: the key steps that your EMS provider will go through to create the perfect oven profile to ensure your products are being built to a consistently high standard.
If you’re interested in finding out more how an EMS provider goes about setting up and controlling their reflow ovens then feel free to contact a member of the JJS team. You can also subscribe to our blog for regular updates on EMS industry news.
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