Machine Monitoring or Process Monitoring?

By Bjorn Dahle, President, KIC, San Diego, CA

In a world of more and more data, we need to choose what to monitor. Do we focus monitoring around a particular machine or do we monitor a process regardless of machine characteristics?

In this article we explore both options and consider which approach will ultimately deliver the greatest value in terms of quality, yield and cost benefits, whilst supporting a holistic data strategy. As with most of these decisions the answer is rarely as clear-cut as ‘this one is better than that one’. The first thing to consider in any selection process is the desired outcome. Is there a specific issue you need to resolve or are you looking to simply monitor trends? Are you looking for a traceability solution dictated by a customer or a standard? Are you wishing to monitor, and consequently improve the performance of your own line and processes or are you wanting to check that a particular machine stays within the specification it has been set up and commissioned to deliver?

This article is based on my experience with SMT reflow ovens, but the principal of what to monitor applies throughout any connected data driven process. Within the reflow oven there are numerous variables that can be measured: heat developed, power consumption, vibration, nitrogen consumption and many more. Selecting the right measurements is essential in monitoring any performance and thus is the starting point in the selection of your monitoring process.

If you are dieting you would probably monitor you weight, but the truth is most people diet because they want to be slimmer and not because they want to weigh less. If your regime includes working out, you’re probably building muscle that weighs more than the fat you’re burning and as a result you could easily lose inches and not pounds, so why not measure inches?

The same is true in the case of the reflow oven. If we can only improve what we can measure, we should at least be mindful of what we want to improve! The reflow oven’s primary job is to heat solder paste to a point where it reflows and produces good conductive joints throughout the PCB. This requires more definition from all the vested parties, such as solder paste vendors, laminate suppliers, component makers, but in essence good solder joints are what we are asking our oven to produce. The way we currently measure that success is by using something we term as a ‘process window’. This is the agreed acceptable process tolerance range within which we get the result that all the parties agree we should have. Within the process window our solder reflows and creates good reliable joints. Within the process window our components are not damaged or compromised. Within the process window the substrate remains intact.

To achieve the desired process window we need to have a temperature profile. This is the profile of the time spent at each given temperature as the PCB passes through the oven and this is governed simply by controlling conveyor speed and zone temperature or heat transfer.

The oven thus has a simply defined role to play. It must operate at production speed applying an acceptable profile to each PCB.

In an ideal world we’d just measure solder joint quality but since we cannot, we can only measure those things that contribute to that. There are a number of variables that contribute and can be measured. Some like incoming PCB temperature or thermal mass vary little and have minimal impact so can be ignored, whilst others like zone temperature, air-flow and static pressure have more impact.

Having decided what we want to measure we next need to consider how best to make that measurement, where should we get the data and how should we process it?

Let’s start with a couple of definitions. Machine monitoring first of all, does what it says on the tin, it monitors the machine performance. We know we need to monitor zone temperature, air-flow rate, conveyor speed and static pressure and if we monitor these within the machine we can see how well that machine performs against the specification it has been set up to deliver. We can see when it drifts out of tolerance and we can decide how to alarm the system and what corrective action to take.

This is perhaps the path of least resistance. If you have a process that works and you’re happy with, you can monitor against that, decide what is and isn’t an acceptable variance and work from there. You don’t even need to know your process profile that well, if you have a set up that works for a wide range of products, you can set that as the benchmark. This is without doubt the simplest and least expensive option, but it does come with limitations.

Firstly machine monitoring assumes that the four variables are all you really need to know, regardless of whether the profile is ideal for the PCB, this may or may not be true. An oven is continuously self-correcting, cycling up and down to maintain zone temperature against other variables such as thermal load. It may also be using power saving strategies like variable blower output or sleep modes that will complicate monitoring considerably.

So, what is process monitoring and how does it differ?

In simple terms rather than measure what’s going on with the machine, process monitoring will measure what’s going on with the product when it passes through a machine.

If you customers are dictating you run a particular profile then you’ve probably already adopted process profiling, since it’s the only way to provide them with a genuinely traceable profile measurement. Process monitoring is delivering three data sources: the detailed measurement of the PCB profile; this profile’s ability to accommodate the agreed process window; and the change or decay in that profile over time.

As a result you measure what you need to know – how the process is performing. By measuring this you can improve the process to get better results, reduce scrap, increase yield and lower costs. For example you might see that rising profile slope, or the time above liquidous is getting close to the edge of the process window offering clues for process improvement.

You will also get valuable SPC data as well as full traceability of the process. This means everything from monitoring the performance of the machine against the profile you’ve set to whether or not an operator has loaded the correct oven program for each product that comes down the line.

While process monitoring offers clear advantages over machine monitoring it does not come without demands upon resources. To monitor process you first need to know your process window for each and every assembly coming through the oven. To give 100% traceability you’ll need to monitor much more than occasional samples. And the systems required for process monitoring are more sophisticated, so come with additional costs.

So, in conclusion, these are two different solutions, resolving two different issues. Machine monitoring monitors the variable within the system that impact on an outcome, where process monitoring monitors the outcome. Selecting the right solution is simply about selecting what you want to learn. Are you concerned that your machine specification will drift over time? If so, machine monitoring will do that. If you are concerned about the quality or the solder joint and its reliability in the long term as well wanting to deliver complete traceability and process improvement, then process monitoring will do that.

To go back to the diet analogy again, if you want to be slimmer you’re better off with a tape measure than scales. If you want to lose weight get scales.
Reprinted with permission from Global SMT & Packaging, August 2015.