Split-second soldering
October 2009
by Andreas Hohnerlein, Inertec Löttechnologien GmbH
Andreas and Ernst Hohnerlein are meeting new demands in industrial soldering with their patented process for matrix soldering under inert gas. They developed the process and designed a machine suitable for industrial applications.
Large plugs or mechatronic components on today’s state-of-the-art printed circuit boards represent major challenges for industrial soldering processes that must withstand the rigors of mass production. For a long time, sequential wave and mini-wave soldering approaches dominated – with one devastating consequence: the larger the component or packing density, the longer the soldering cycle time. But soldering in synchronization with the production line means shrinking the cycle time for the entire workpiece to 25 or 30 seconds.
Inertec’s latest generation of selective soldering systems meets the heightened requirements of automotive suppliers regarding mass soldering systems – particularly with respect to selective soldering. Our patented matrix soldering system is used in three-shift operations around the globe and is designed to withstand heavy use. The system has to be able to deal with all the situations arising from upstream thermal processes (warped modules), and for this reason is provided with some special design features.
This machine, newly developed in 2006, consists of flux, preheating, soldering, cooling and stacking stations arranged in a U configuration. Thanks to dynamics-oriented automation, the plant achieves the required cycle time of just 25 to 30 seconds. Most of this time is used for soldering. Rexroth linear motion, drive and control technology and pneumatics keep non-productive periods short. Why we chose Rexroth is easily explained. For one thing, we share a long history of partnership. But even more, Rexroth is able to provide single-source solutions, drawing on several fields of technology, using linear motion and pneumatic components along with the required control elements.
Our matrix process greatly accelerates the soldering of electrical circuits with plugs, tall components and mechatronic modules that have already been mounted in a housing. The geometry of the special soldering matrix matches the soldering points on the bottom of the circuit board. Fingers at those points are topped with a cup that can carry a drop of solder. The matrix is submerged in the lead-free, liquid solder so that the cups are filled. The matrix is then raised until contact is made with the soldering points. Until matrix soldering was developed, this step had to be performed manually or by a robot. But soldering individual joints one at a time, in sequence, was seriously out of step with modern fabrication concepts. By contrast, our process solders all the joints at one go. It also lets designers increase
component population density.
Simultaneous motions
Inertec has realized this process in a highly productive machine concept. Workpiece carriers with standardized outside dimensions transport the workpieces – borne in component-specific templates – through the entire system. The actual process begins at the flux station. Here, stamps configured to match the soldering geometry moisten the soldering points. The flux applied here improves the quality of the bond. A Rexroth TRR pneumatic rotating cylinder on the top of this station simultaneously moves both the hold-down device and the matrix for the flux. This is done to ensure co-planarity, something that is key to component quality, since thermal deformations can occur during the upstream process steps. Four corrosion-resistant Rexroth shafts with two ball bushings each guide these simultaneous motions. Together with the shafts, which are supported at both ends, the low-maintenance, easily mounted ball bushings act as cantilevered linear guides. Inertec uses segmented ball bushings at the flux station. In this rust-free construction the guide cage is made of polyamide while the balls, segments and metal retaining rings are engineered from corrosion-resistant steel.
Rapid handling
From the flux station, the transport system moves the components through the preheating station at fixed timing. There the PCBs are gradually brought up to the optimum process temperature without damaging the sensitive components. At the entrance to the soldering station, a servo shaft takes the workpiece carrier. Four gripping jaws actuated by a type OCT pneumatic cylinder take the workpiece holder and place it in the soldering station. The soldering matrix rises upward through the template, making contact with the soldering points, while at the same time a hold-down device ensures the co-planarity of the workpiece and keeps critical components in place. A skimmer cleans oxide residues away from the surface of the solder pool before the next soldering cycle begins under the inert gas atmosphere.
The stamp is moved using a Rexroth servo drive via a ball screw assembly. Inertec moves the stamps outside the machine using two Rexroth shafts with two ball bushings each. After the soldering step, a servo shaft takes the workpiece holder and transports it sideways to the cooling station. This motion uses two Rexroth Compact Modules for the Z and Y axes. These are made up from an extremely compact precision aluminum profile rail with a sheet aluminum cover. The gap at the circumference is sealed with polyurethane tape to protect the internal components.
For the long Y axis Inertec uses the belt-driven CKR Compact Module to minimize non-productive times. Using a standard connection plate with the Easy-2-Combine feature, a further CKK Compact Module with ball screw assembly is attached in a positive, no-adjustment connection for the Z axis. A centrally located ball screw assembly positioned between two ball rail guides reduces the unit’s height. The ball screw assembly with zero-play spindle nut ensures high positioning accuracy and repeatability.
The cooling station takes the workpiece holder directly from the gripper. The machine buffers the cooled components in a stacking station and releases them on a first-in-first-out basis. In this way we ensure that components are forwarded in the correct sequence in the event of product changes while the system is running.
Perfectly matched components
The Inertec machine uses three Rexroth servo drives, coordinated by a superordinate Rexroth IndraLogic L40 controller. The controller-based hardware communicates via Ethernet with the Rexroth IndraControl VCP 20 compact operating panel. The Rexroth NL maintenance unit prepares the air for a total of ten pneumatic cylinders in the flux (TRR), soldering (OCT) and stacking (RTC) stations. This unit cleans the incoming air using a filter unit with 5 µm filter mesh, and handles the individual air preparation processes – filtering, regulating and drying – in a single unit. The modular design of the maintenance units also means that individual components can be replaced without detaching the entire unit from the system.
Two-minute changeovers
One special feature of this machine is that products can be changed without interrupting production operations. The operator changes the matrix and hold-down device in the flux station while the soldering station works through the four components that are still in the preheating station. Thanks to the quick-change concept, refitting takes less than two minutes, and the flux station is ready for operation after just four idle cycles. Next, the operator changes the matrix, mask and hold-down device in the soldering station. A “loss-proof” coded plug is used to tell the system which component is currently entering the station. After just four idle cycles, the new workpieces, moistened with flux and preheated, arrive at the station. And this process can keep running around the clock.
Author:
Andreas Hohnerlein, Managing Director, Inertec Löttechnologien GmbH
More information:
www.boschrexroth.com/linearsysteme
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