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The importance of reflow profile optimisation and How it can affect modern manufacturing trends

Published: 6 January 2016 - Lisa Peake
 
Mark Stansfield, Managing Director, SolderStar
 
To produce electronic assemblies without a known thermal profile is detrimental, as you ‘cannot manage what you do not measure’. Without an established oven profile, problems can occur with soldering quality, cleaning issues or further down the line involving either rework costs or early component failures.
 
Setting up profiles for each electronic assembly can be a time consuming exercise without the right engineering tools, especially when assemblies comprise of large thermal mass differences or when we have ovens with high numbers of zone count. To help combat this problem, profile prediction tools have been developed to simulate the reflow process allowing ‘what if’ scenarios to be performed. Today advanced profilers and software which provides reflow optimisation have changed the face of electronic assembly by improving process efficiency.
 
Growth in SMART devices has increased the need for intelligent profiling
 
Continuous development for newer technology requires successful PCB assembly but this in turn increases the number of defects. It is very important to achieve good thermal equilibrium across the whole board especially with lead-free soldering since the peak temperature window is narrower. The need to analysis and improve on performance is becoming an uncompromising requirement, but to achieve this, reflow profile optimisation is needed.
 
Since a modern reflow oven has different temperature zones and a variety of conveyor speeds supporting literally hundreds of millions of different oven setups, modern process setup tools are needed to identify the best combination.
 
PCBs also have differing thicknesses and component densities, usually demanding different profiles. By optimising a thermal profile of the new assembly you can be sure that thermal requirements are correct, leading to less failures. 
 
Profile prediction tools radically simplify the setup of reflow soldering
 
There are currently two systems in place that offer measurement control, the first being the standard Profile Seeker. Profile Seeker allows offline evaluation of heater and conveyor speed changes to be made. The utility can save time and money by removing the need to make numerous unnecessary profile runs when a new profile is set-up from scratch.
 

The system shows the effect on the profile by drawing a predicted profile alongside the existing one and also uniquely shows graphically the effect on key process parameters. With so many variables in today’s soldering processes finding the optimum profile can be difficult unless it is very near completion, in answer to this AutoSeeker was developed.
 
Autoseeker searches millions of reflow oven set-point possibilities to find the best possible process settings for these sensitive assemblies. When combined with Automatic Profiling Systems, like the new SolderStar APS it provides a powerful combination of optimal profile setup and 100% monitoring of every assembly through the production line. 
 
The need for reduced oven recipes and faster changeover
 
One of the other major uses of profile simulations is the need to reduce the number of oven recipes being used on the SMT line. Ovens can be slow to stabilise when heater changes are required, sometimes taking 30 minutes to come down in temperature before production can begin. 
 
Recipe harmonising is also desirable for the engineers, allowing groups of assemblies to be processed on the same oven setting successfully. Another common requirement is to find a suitable profile which only requires a conveyor speed change. The correct software tools allow both of the above to be achieved. The AutoSeeker tool has options to constrain the optimiser to, for example, only look for solutions that require speed to change. Individual heaters can also be ‘pinned’ so that preferred profile shapes can be maintained.
 
How do these Systems Work?
 
Early SolderStar software used a simple time constant approach, ‘borrowed’ from the electronics world of simulating a capacitor charging through a resistor. This simplistic method provided results which were useful but not accurate by today’s standards. 
 
For automatic profile simulation we needed more accurate modelling of the process. SolderStar’s latest algorithm uses complex functions which more closely models heat transfer characteristics and yields more accurate profile results.
 

“But how can you determine it is actually correct?” this is quite simple, it can be done in three steps as an exercise,
 
1) Enter a chosen recipe into the oven, allow it to stabilise. Capture a temperature profile from a chosen assembly.
2) Enter a 2nd recipe into the oven, allow it to stabilise. Capture a temperature profile from the same assembly.
3) Now enter the differences from recipe 1 and recipe 2 into the simulator and see what it predicted.
 
The perfect system would yield a result of the real profile captured in step 2 and the predicted profile from step 3. In reality this is not possible, but we can get very close, and certainly near enough to make the exercise a massive time-saver.
 
Finding the Answer
 
Using this method of measuring two real thermal profiles and comparing the results with what the simulator predicted allows us to gain a measure of how accurate the model works. As with all mathematical models that are using real world data, with measurement tolerances and noise present within the system some errors will occur. The results show that with even a considerable change of temperature of 20C across all heated zones the simulation algorithm provides extremely accurate answers.
 
What must be remembered is there is no universal best profile. It cannot be determined by the paste, PCBA or reflow oven, it is a combination of all of these and this is why software that automatically drives and predicts an optimal solution for an individual profile is important. The growth of miniaturisation has boosted the electronics market, but with it has brought profiling problems. By installing software to predict reflow oven optimisation, manufacture of micro-assemblies can be achieved quickly and cost effectively.
Source: Electronics


 
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