TEMPERATURE PROFILING SYSTEM

- A BUYER'S GUIDE

Introduction

The industry is about to undergo major change. Lead free soldering is coming and if you don’t 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 PCB's 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 the staff have?
•  What is the capability of my oven?

The Datalogger

Basic features

The 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 the following 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 manufacture 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 6 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 3 or as many as 16 channels.

There are two main factors which determining 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 4 channels and ideally 6 channels are required in modern processing.

Another area to watch is interference on readings. Thermocouples act as excellent antennae coupling high frequency signals into the measurement system. Low frequency power line interference can also be troublesome.

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 220C. Hotter lead free process may cause problems here.

Data Storage

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 powered down or the batteries are removed for replacement or charging.

On systems that operate with a single START/STOP switch, it is easy to accidentally overwrite data by accidentally pressing the switch twice on a STOP (accidental restart). Some systems have a separate START/STOP button or have a built in feature to stop this happening.

Data download

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.

The following points should however be noted with such systems: -

• Most systems use a PC COM port to download data to the datalogger. Newer PC are increasingly becoming 'legacy free' meaning that the COM ports and printer ports are sometimes not fitted. USB is increasingly being used so its worthwhile checking the system is compatible or can be upgraded.
• Check that the connector system used on the download lead is of good quality. The datalogger will continuously be plugged on and off, 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 processes can last upto 10 minutes long. It can be infuriating to wait upto a minute to download your data.

Some more expensive systems allow realtime viewing of the data, for example by offloading the data over a RF link.

This type of system offers the following 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 )

Unfortunately there are downsides: -

• The systems are relatively expensive.
• Despite what the manufacturer may tell you, all systems suffer performance problems caused by interference and other environmental phenomenon. Check to see the system will work in your factory before buying.

Durability & Protection

The working environment for the datalogger is a harsh one. Thermal cycling of electronic equipment is a long accepted method of accelerating its life. It therefore goes without saying that the thermal cycling of the

datalogger itself should be kept to a minimum i.e. some form of heatshield should be used.

In addition, systems which do not employ a separate heat shield are potential 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.

Minimising the size of the datalogger allows the maximum amount of insulation to be used in the heatshield for a given overall size constraint.

The increasing process temperatures used in lead free soldering are likely to mean that all systems in the future will require a separate heat shield.

Specifications & 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 0.1C 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

Ongoing costs will include some or all of the following: -

• Software maintenance

Check to see if upgrades are free or chargeable. Some systems offer upgrades for a limited time only.

• Repair costs

It pays to check typical repair charges. Systems with rechargeable batteries are more susceptible to damage as they have a lower maximum  working temperature. Check the cost of replacement and the expected

life.

• Calibration costs

Calibration should be performed every 6 or 12 months. Check arrangement for this. Can your own Calibration 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: -

Basic Features

• Capture, download and view a temperature profile.
• Save and Print data.
• Measure basic process parameters using cursors.
• Compare a profile with one previously captured.

Advanced Features

More expensive system offer a wealth of features which a prospective buyer may or may not require, these include: -

Profile Prediction

Adds the ability to simulate the effect of machine setting changes on the temperature profile. Systems fall into two categories. Manual systems, which allow the oven heater settings and conveyor speed settings to be previewed.

Automatic prediction calculates all process combinations, then provides the user with the optimum setting.

SPC

Allows automatic gathering and analyses of key process parameters for the purposes of SPC. These features are normally 'add-on' modules to the profiling software; some systems also ease the capture of data for SPC

analysis in 3rd party SPC software or a simple Excel spreadsheet.

Automatic Parameter Calculation

Most systems will automatically calculate process parameters from the data you capture. This feature saves time and is the only quick way to compare one profile with another. These parameters are often presented

in a spreadsheet style table; some newer systems also present these graphically to expedite the analysis of the data.

Quality Control Documentation

Quality control is often the driving force behind investment in a temperature profiling system. It is the only sure way to gain full quality control of your soldering process.

All systems allow a printout of the data you have captured, many newer system will produce all the information you require for documentation of the process to be included within a test report, i.e. Oven zone setpoint's,

PCB details, all process parameters.

An IPC standard for temperature profiling is now available for purchase at

www.ipc.org. (IPC-7530)

Machine and Solder Libraries.

These allow the user to quickly setup the system by picking paste parameters and machine settings from a pre-configured library. This feature will save you time and remove errors entering data to the PC

software.

Compatibility

Check that the software PC software is compatible with you operating system and target machine.

Many suppliers will offer demonstration software for you to try before purchasing a system.