Bringing, and keeping, machine soldering under control

by

Mike Hayward

The profitability of an electronics factory can be greatly influenced by the quality control tools and methods used for data collection. This is especially true of the soldering operation.

Inspection, rework and excessive maintenance procedures all add to manufacturing costs and it is therefore vital to minimize occurrences of such events. This can only be achieved by ensuring that soldering equipment, either wave or reflow, is operating within required specifications.

The initial step for any machine soldering operation is to ensure that the correct recipe is used for the combination of materials, chemistry and machinery. This is usually referred to as "Profiling", the end result of which is a set of process parameters that give the best possible soldering result for a given combination of ingredients. (Fig 1.) It should also give the optimal process window so that minor fluctuations in any of the parameters will not have a detrimental effect on product quality. (Fig 2)

As stated above, the establishment of the profile or recipe, is only the initial step towards a controlled process. To ensure ongoing repeatability a method must be adopted to gather, process and present the necessary data in a way that can be easily interpreted and understood.

Statistical Process Control (SPC) has long been established as the perfect solution to this problem. Properly used it can help a process perform consistently and predictably. It can be used to distinguish between normal and abnormal variations that determine the actions needed on a system. SPC control charts can also provide a common language and graphical means for discussing performance.

Until now, perhaps the biggest drawback with solder machine SPC has been finding a reliable means of collecting the necessary data. The harsh environments, such as high temperatures, chemical attack and thermal shock, encountered in machine soldering have always made this difficult. The only way of establishing the repeatability of a process has been to employ sample boards with thermocouples attached, just like at the profiling stage, or a purposely made "test" pcb of a similar material.

By their nature these sample and test boards are susceptible to machine environments and as such are variable. After a few passes their physical characteristics change resulting in different amounts of heat absorption, components may detach causing a change in mass and thermocouples may need replacing. These factors mean that the results measured will have a variation, but in order for SPC to be effective it is important for the data collected to be of a high integrity. If it is not, it will be impossible to tell whether a variation has been caused by the process under investigation or by the tool being used to measure it! Also in some cases, the pcb used for profiling may have a very high value, which when combined with line downtime as a result of profiling, can make the whole operation very expensive indeed.

Thermal Profiling manufacturer ECD, has with the SuperM.O.L.E. Gold platform developed a solution for both set up and SPC control of reflow and wave soldering machines. The modular hardware concept utilizes the same thermal profiler, the SuperM.O.L.E. Gold, in both WaveRider and OvenRider pallets as the data collection system used for machine control measurements. The WaveRider or OvenRider pallet (dependant on the process being measured) replaces the sample or test board, and so removes the variations these can cause. Both pallets are completely self-contained, with no trailing leads, and are designed to collect both thermal and dynamic information as they pass through the respective machine.

Manufactured from a very stable ESD material, the WaveRider and OvenRider pallets are capable of withstanding hundreds of passes through a machine. Each is equipped with sensors that measure the thermal changes throughout the process and motion detectors are used to calculate speed. Each group of sensors is positioned to monitor a specific section of the machine; thus a picture can be built up of machine performance from front to rear. (This is especially important for the wave height, contact length and contact time of a wave-soldering machine – are they the same across the whole machine?) In reflow terms, the OvenRider can determine whether the same temperatures and heat transfer rates are being delivered across the whole width of the machine – true delta T measurement.

The conversion of data collected into meaningful results has also, until recently, been very time consuming and cumbersome. Most established methods using sample boards require data to be downloaded to

the profiling software, this must then be exported to either a spreadsheet or text format, which is in turn imported into either an SPC software package or custom written spreadsheet. Often judgments have to be made that are at best subjective; this requires a high skill level and can only be done reliably with years of experience. This is especially true with wave soldering. Also, by only measuring the thermal characteristics, as with profiling, we are not taking into account the effect of the wave dynamics and the transport system. Both interact significantly and solder results will be drastically effected if we do not consider this.

The SPC software programs available from ECD have been specifically designed to be both easy to use and understand, whilst at the same time delivering the information needed for process staff to keep control. A common software approach has also been used which provides the same user interface and tools for WaveRider SPC and OvenRider SPC – if an operator knows one, he knows them both! Very important in factories where both soldering methods are used.

Data transfer to the host PC is a single click operation. All archiving and processing is fully automatic. Process controls charts, in X-bar and Range formats (Fig. 3) are drawn automatically. A color coded data sheet, which compares measured results to specified engineering tolerances, tells the operator instantly if his process is within specification, simply by changing the color of any suspect parameter. A further standard feature is the calculation of Cp and Cpk indices for chosen parameters. These are capability indices used in defining how capable a process or machine is of producing repeatable results. They are used in conjunction with X-bar and Range charts. (Fig. 4)

The speed at which this data is available allows the machine to be handed back into production, or corrective actions to be taken with absolute minimum downtime.

Powerful tools and search functions enable the user to report performance as part of their normal quality control system. Should the soldering operation be brought into question as the a result of product quality concerns, a very quick search of the database will eliminate the process under question from suspicion and allow production to continue without the need for extensive, and expensive, investigations.

The SuperM.O.L.E. Gold Platform is the only system available which gives access to both the set-up and SPC control of Wave and Reflow machines. Gone are the days of setting up complicated test boards, spending hours with data input, not being sure if the results are really correct. The OvenRider and WaveRider are designed to bring and keep machine soldering under control with the minimum of effort.