Secrets and benefits of oven engineering

Terry Morgan, Parkheath Ltd.

The dramatic improvement in process quality that a high-class reflow oven can bring to any assembly line is routinely underestimated. Reflow machines are, instead, regarded as commodities with little or nothing to distinguish one vendor’s equipment from another (and no shortage of vendors either). This misleading assumption probably stems from the fact that, in essence, a reflow machine seems very simple: a conveyorized furnace that is designed to heat solder-paste alloy above its wetting-point temperature to form a set of soldered joints on a board assembly.

But in reality, ensuring that all components, terminations and PCB pads are securely brought to reflow temperature at the same time is anything but simple due to wide variations in thermal mass (or heat absorbing capacity) of the differently sized and colored parts. Yet, if a board is not brought up to reflow temperature in a gradual and controlled manner, components with a smaller thermal mass can rapidly overheat and suffer permanent damage. Similarly, smaller deposits of solder paste can also be ‘overcooked’. This increases their required soldering time by prematurely burning off flux before it can do its job, and promoting oxidization that hampers good quality joint formation. Such oven-related process failings can lead to unacceptable large variations in soldering quality. Although many users attempt to reduce such problems by making trial-and-error modifications to their reflow profiles, the same set of problems tends to reoccur with each new board. This makes it difficult to accommodate board designs.

The good news is, however, that almost all reflow-related defects can be eliminated by the use of a high quality oven. On the other side, too many assembly defects and repeatability problems develop directly from a poor choice of reflow machine even though some users are unwilling to admit this fact. In addition, the increasingly diverse and dense range of small components used in standard electronic products means that maintaining an effective reflow process is becoming more challenging. This is because such boards demand more gradual and controllable heating profiles than many existing ovens can deliver. This means that suppliers and their local support such as representatives need plenty of experience to overcome all the process problems that a poor choice of reflow oven can cause. A compe-tent machine supplier, therefore, focuses huge amounts of R&D effort into reflow machines that are designed from the outset to totally eliminate such difficulties.

This has culminated in the development of Rehm’s highly advanced SMS-V6 standard platform upon which all its ovens are currently based. Primarily aimed at higher volume reflow requirements, this platform forms the basis of fivedifferent machines offering successively higher throughput rates (or reduced typical cycle times). These currently range from around 94 PCBs per hour (40s cycle time) on the SMS V6-2100 up to 231 PCBs/panels per hour (15s) on the SMS V6-4100. In addition, all ovens are equally suited to soldering under air or nitrogen, as well as adhesive curing applications, and can reliably accommodate the complete spectrum of SMDs including µBGAs, flip chips and fine-pitch devices such as complex QFPs.

To accommodate lower throughput and reduced floor space needs, the scaled-down, but nonetheless SMS-V6-derived, Compact 1600 and 2000 series ovens that measure 1.6 and 2m in length respectively are also available. These are complemented by an ultra compact VRS vertical reflow furnace that offers a high volume throughput (20s cycle time) in a footprint that takes up around half a conventional oven’s area. As a result, any of those ovens benefit from the same leading-edge technology. A powerful platform-wide modular construction also allows all ovens to be precisely customized to meet the individual needs of any user, and makes future upgrades extremely straightforward.

In order to demonstrate why Rehm’s ovens cannot simply be classified as commodity products, it is necessary to examine their thermal and electro-mechanical performance, build quality and advanced processing capabilities in detail. To guarantee optimum thermal process stability, all these reflow ovens exploit forced hot-air convection to transfer thermal energy quickly and efficiently to PCBs through preheat, soak, reflow and cool-down phases. Powerful fans are used to blow large volumes of heated air onto one or both sides of the boards, and to make this air sufficiently turbulent to ensure reliable heating penetration into even the smallest gaps on an assembly. The circulated air is heated by high-grade heating elements and accurately directed from the fans back into the process chamber via specially designed, thermally segregated, nozzle fields. These nozzles are also used to create forced hot-air convection gas-flow characteristics that ensure components are brought to reflow temperature simultaneously and delta T (temperature differences) across boards are minimized.

To further ensure stable and reliable soldering of any component type, each heating zone on the SMS-V6 platform is independently regulated, and the volume of convected air is precisely controlled via fine adjustments in fan speed. For ovens with 3300mm of heating length upwards, we also employ double-peak heating zones as standard. These allow increased heating capacity and circulated convected air volumes. This gives users maximized flexibility, even soldering extremely large assemblies. Although the heating system employed in all ovens is equipped with four or more convection fans, additional fans and nozzle fields can be integrated where required above and below the transport level to increase the operating range of a system. Supplementary IR-heating radiators can also be added if necessary.

Cooling boards after reflow is also important because faster cooling increases throughput and reduces the common problem of dry joints caused by components moving during post-reflow handling. Assemblies are cooled by a controlled ramp air-blow system using nozzle fields arranged above and below the conveyor system. Cooling can also be used in the peak (reflow) zone heating section below transport level to allow the processing of double-sided boards and prevent previously soldered joints on the underside of a board reflowing a second time. This technique can maintain a temperature difference between the top and underside of a board to a maximum of up to 20 or 30ºC. Rehm’s thermal technology allows its ovens to offer an unusually large heating capacity and to accommodate solders with melting points up to, or in excess of, 220ºC. This means that they can immediately accommodate the higher temperature requirements of lead-free soldering without need of any re-configuration.

To achieve the optimum heating cycles or profiles for a given board, all ovens exploit powerful profiler hardware and SPC software to accurately monitor temperature variations across a board. This gives users the ability to continuously control and automatically fine-tune their reflow profiles. The air temperature in each heating zone is also measured via sensitive thermocouples situated next to the conveyor system. As a result, once a heating profile has been established, the furnace will hold it exactly. To guarantee optimum electro-mechanical process stability, the equipment is also built from high-quality materials, and, as a result, exhibit a class leading mechanical integrity. Although oven vendors strive to ensure their machines offer full air and temperature segregation between zones, some machines do suffer from a certain amount of bleed-off at higher processing speeds, which can have a serious impact upon soldering performance. To prevent this, the process chamber is manufactured out of stainless steel, and insulated externally against heat loss by extremely high-grade fiber panels. It is also totally air tight when operational.

One of the primary distinguishing features of all Rehm ovens compared with its competitors is the application of a water-cooled gasket to seal over the upper andlower sections of the process chamber’s clam shell. This maximizes both the lifetime of the product and air-tightness of the seal with respect to the external environment. It also produces very low heat dissipation to the shop floor and, most importantly, enables users to greatly expand the size of their process windows within the heating chamber. In addition, the process chamber itself is very narrow with the sidewalls brought in and positioned very closely to the sides of the transport conveyor. This further helps minimize thermal dissipation during processing across the conveyor width and produces an extremely even spread of heat across each board.

These features are further supported by a very robust and reliable transport system which includes a pin-chain conveyor with (optional) PC-controlled, top quality ‘infinitely’ wired Kevlar-rope center-support, to prevent board warpage. For the pin chain conveyors, a continual monitoring system is also used, so that in the highly unlikely event of a board falling down the operator will immediately be alarmed. All board transport options can be combined in a dual-lane format if required. Rehm is the only reflow-oven manufacturer who can supply a machine to the full dual-lane SMEMA standard. This means users can have two synchronously positioned lanes, 35mm apart, that are independently controlled but balanced to optimize board flow and machine utilization. This arrangement can produce enormous throughput and productivity gains.

A final area where these ovens stand out from the competition is in terms of their efficiency in waste management of by-products from the heating and soldering process. This results in dramatically reduced maintenance demands, lowering cost of ownership. Although forced cooling offers many process benefits, it can create maintenance problems due to atmosphere-borne flux contaminants condensing in the cooling zone and leaving an unpleasant sticky residue within the machine that is time-consuming to remove. In the Rehm ovens, this is combated via the standard use of an extremely effective and fully integrated flux management system. On the SVS-V6 platform, this comprises a three-step water-cooled condensation trap in which flux vapors and volatile materials are condensed after being extracted from the reflow atmosphere between the peak (reflow) and cooling zones. The air is then re-introduced into the oven to maintain a clean atmosphere and an efficient process that produces high-quality solder joints and dramatically reduces the amount of maintenance. In fact, the interior of these ovens requires hardly any cleaning at all, and customers are always remarking how clean the machines are in operation. So instead of having to regularly allow the oven to cool down in order to clean it, users only have to spend ten minutes removing and replacing a set of filters that can be regenerated for future use.

The breadth of the Rehm ovens range has allowed even more effective partnership with customers, enabling the development of highly catered solutions that perfectly meet the specific requirements of their individual applications. As a result, our customers can be assured that we have the ability to identify and deliver the best working reflow solution for their requirements. However, such performance and quality doesn’t come cheap. But such a reflow machine is the equipment of choice for many users, especially those in volume assembly. And in terms of cost, the increased line yield, uptime, reduced scrap and waste levels, smaller maintenance requirement and ease-of-use all combine to ensure that such a system will pay for itself over the lifetime of its use. In this case, customers have decided to select the peace of mind of having total reliability and quality. And such a reflow oven has equal or improved cost-of-ownership than any other major capital equipment on a line, but has exactly the same impact on process quality. As a result, almost every user could benefit from such a system, provided they recognize how significant an impact it could have on their process.

Eine große Zahl von Ofen-Anbietern drängelt sich im Markt und augenscheinlich kann man mit vergleichbarem Equipment auch alle ins Auge gefaßten Reflow-Aufgaben lösen. Lötanlagen für Baugruppen werden damit bei dieser Sicht zu einer Art von leicht vergleichbarem Standardprodukt. Der englische Repräsentant von Rehm aber sagt, wesentliche Unterschiede liegen im Detail und machen sich in der täglichen Praxis des Anwenders erheblich bemerkbar

De nombreux fabricants de fours se bousculent sur le marché et apparemment, toutes les tâches de reflow peuvent être réalisées à l’aide d’équipements comparables. Vus sous cet angle, les équipements de brasage pour circuits deviennent une sorte de produit stand-ard facilement comparable. Mais le représentant anglais de Rehm affirme que les différences majeures se situent dans les détails et se font fortement remarquer dans le cadre de la pratique quotidienne.

Sul mercato attualmente si sta insediando un gran numero di offerenti di forni e apparentemente con equivalenti equipaggiamenti si possono anche risolvere tutti i problemi di riflusso che danno maggiormente nell’occhio. Con ciò gli impianti di saldatura per gruppi di costruzione in questo aspetto possono essere considerati come un prodotto standard facilmente paragonabile. Il rappresentante inglese della Rehm sostiene però che nel dettaglio vi sono delle notevoli differenze, che si manifestano sensibilmente in particolare nella pratica dell’utente.