The industry is about to undergo major change. Lead-free soldering is coming and if you do not profile your process now, then you soon will be. This guide is aimed at explaining the elements and features of typical profiling systems especially for people new to profiling.
Which features do I need?
Before considering any of the temperature profiling systems available, it is worthwhile spending a little time considering what your requirements are. All of the systems offer at least basic functionality, some of the advanced features are optional, so you should decide which if any you require. Consider the points below to help you decide.
* Do the type of PCBs produced change often?
* Do the boards vary radically in density?
* Are the boards a complex mix of different technologies?
* Do I have problems with processing at the moment?
* What process knowledge do you or your staff have?
* What is the capability of my oven?
The datalogger
A datalogger is an electronic unit, which records readings from thermocouples attached to a test board. Some systems have fixed sampling rates and channel configurations but most professional units have these minimum features:
* Some means of selecting which of the available measurement channels are to be recorded.
* Some means of selecting at what rate readings are to be taken and stored.
There are a few important points to be aware of here:
If the measurement channels are sampled too slowly, then the profile you record may not be an accurate representation of the real profile. Conversely, if you sample too fast, then there may not be enough space in the datalogger's internal memory to record the whole process. Faster sampling rates are required when profiling wave soldering processes due to the rapid rise in temperature as the board travels over the solder wave.
Dimensions
This is more important than you might think. Many modern ovens feature reduced entry height above the conveyor. This is done by the manufacturer in a bid to increase the efficiency of the machine and reduce losses at the oven entry. The situation is usually more acute on nitrogen-inert systems. It is therefore important that you consider any restrictions your equipment may have.
Many modern boards, especially on high volume lines, are no longer panelised. Problems can be encountered with some profiling systems where the thermocouple channels are arranged horizontally. On systems with say six channels so arranged, for example, the width of the datalogger plus heatshield can often be wider than the board which is to be profiled. Often the manufacturer will offer a thinner system with a reduced number of channels, the result however is that two systems are required to cover all eventualities.
Measurement channels
Systems on the market are available with as few as three or as many as 16 channels. There are two main factors which determine the number of channels you might want to consider:
* Smallest and largest size of panel you produce.
* The mix of components and complexity of the panels you produce.
Generally speaking, the more complex the mix of components (different size and mass) and the larger the boards, the more channels you will need. It is generally accepted that a minimum of four channels and ideally six channels are required in modern processing.
Another area to watch is interference on readings. Thermocouples act as excellent antennas, coupling high frequency signals into the measurement system. Low frequency power line interference can also be troublesome.
Additionally, some systems use non-standard thermocouple connectors. Whilst allowing the system to be made smaller, this means that you the buyer have only one source for thermocouples. Several systems do not protect the thermocouple connector system with the heat barrier system. Check the spec on the thermocouple connectors used, many are rated only to 220°C. Hotter lead free processes may cause problems here.
Data storage and download
The datalogger's memory size dictates the total number of readings that can be made before data is offloaded. The memory should be nonvolatile. That is, readings and settings should be retained in memory even when the unit is off or the batteries are removed for replacement or charging.
Most of the low cost systems simply store the data in their internal memory for download to a PC at the end of the run, usually over a download lead provided with the system. Consider these points:
* Most systems use a PC COM port to download data. Newer PCs may not have COM ports or printer ports. USB is increasingly being used, so it is worthwhile checking on this.
* Check that the connector system used on the download lead is of good quality. The datalogger will be continuously plugged in and out, every time a profile is captured. Budget connectors can have a mating life of as little as a few hundred operations.
* Check how long it takes to download the data to the PC. Some data downloading can last up to 10 minutes.
Some more expensive systems allow realtime viewing of the data, for example, by offloading the data over an RF link.
This has the benefits:
* Data can be viewed in realtime (useful if your process is long).
* The datalogger's memory effectively becomes infinite (as data is continuously downloaded).
Downsides are that the systems are relatively expensive and may suffer performance problems caused by interference.
Durability and protection
The working environment for the datalogger is a harsh one. Thermal cycling of electronic equipment is a long accepted method of accelerating end-of-life. Therefore thermal cycling of the datalogger itself should be kept to a minimum ie, some form of heatshield should be used. In addition, systems which do not employ a separate heatshield are potentially dangerous to personnel trying to download the data. It is preferable to unload and leave the heatshield in a safe shopfloor location, whilst the relatively cool datalogger is taken to an office PC for data download and analysis. The increasing process temperatures used in lead-free soldering are likely to mean that all systems in the future will require a separate heatshield.
Specifications and jargon
There are three parameters the buyer should be aware of:
* Accuracy: this is the degree to which the system agrees with some independent standard. Make sure you ascertain whether the stated accuracy refers to the system accuracy or the accuracy of the datalogger alone. Most systems specify an accuracy of ±1°C.
* Resolution: this is the smallest change in temperature that can be measured by the system. Here, smaller is better. Readings from a system with poor resolution resemble a 'staircase'. A resolution of 0,1°C is adequate for most purposes.
* Repeatability: This is the ability of the system to produce the same reading if the same temperature is applied. This is an important feature if comparisons are to be made between readings obtained at different times.
Ongoing costs
* Software maintenance: Are upgrades free, chargeable, or for limited time only.
* Repair costs: It pays to check typical repair charges. eg, systems with rechargeable batteries are more susceptible to damage from temperature extremes.
* Calibration costs: Calibration should be performed every six or 12 months. Can your own Cal-lab do this or does this have to be done by the manufacturer?
PC software
Most modern systems come with PC software, which allows data to be downloaded to a PC. The functions of the most basic systems are briefly as follows:
* Capture, download and view a temperature profile.
* Save and print data.
* Measure basic process parameters using cursors.
* Compare a profile with one previously captured.
More expensive systems offer a wealth of advanced features which may or may not be needed. These include: Profile Prediction; Statistical Process Control (SPC); Automatic Parameter Calculation; Quality Control Documentation; Machine and Solder Libraries.
Finally, check that the PC software is compatible with your operating system and target machine. Many suppliers will offer demonstration software for you to try before purchasing a system.
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