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A Selective Wavesoldering Alternative
December 31, 1969 |Estimated reading time: 7 minutes
This new technology provides an alternative to the current, limited options in the area of selective wave soldering.
By Jonathan Wol
The ability to perform selective soldering is becoming necessary for companies that produce mixed-technology printed circuit boards (i.e., boards with both surface mount and through-hole components). Not long ago, the best choices for soldering through-hole parts were limited to hand soldering, palletized and masked wave soldering and the use of multi- and single-tube nozzles with dedicated selective soldering equipment. Today, these methods face several limitations:
- Hand soldering often produces upward of 1,000 defects per million opportunities (DPMO). Defects range from insufficient solder joint fill because of varying dwell times to cold or missed solder joints. Yet, even though it is hampered by defects and is labor-intensive, hand processing remains the most widely used method for soldering through-hole leads on heavily populated, double-sided circuit board assemblies.
- While wave soldering offers the benefits of a controlled process and reduced defects, it is limited by the need to implement specialized pallet fixtures that mask areas from unwanted solder contact. Additionally, changes made to the process often necessitate hard tooling changes in the pallet, resulting in a slowed development process and increased startup costs.
- Current selective wavesoldering machine designs offer either 1) single-point selective soldering to control the process joint by joint, or 2) multi-dip nozzles to accommodate several points at once. Some users, however, are encountering a "gray" area that may negate the benefits of both methods, (i.e., the multi-dip nozzles are unable to provide a consistent process, while the single-point alternative, though consistent, cannot deliver the required production rate). Single-point can dictate a different process for each pin but cannot cover large numbers in one dip.
"Tailoring" the Part to the ProcessA recently developed nozzle, called the Jetwave, is said to provide a cost-effective alternative. While affording the benefits of a high production rate, the high-velocity Jetwave nozzle also matches the process control and programmability advantages of the single-point unit. Jetwave technology is designed to overcome the processing problems presented by the odd forms of connector design. For example, the ratios of pitch-to-pin length, pad-size-to-pin length and pin-size-to-hole-size, plus pin shape and other factors can combine to make the multi-dip process difficult to control. The advantage of the Jetwave nozzle in these circumstances is its ability to provide fast cycle times due to three factors: a large contact area, free programmability and a capacity to move the board through the wave rather than perform a straight dip.
The board motion eliminates bridges that would be evident in a straight-dip process. The control of a board's angle as well as its rotational orientation permits the process to be "tailored" to each component or connector. Thus, it is similar to the wave process with masking but with the added advantages of no fixed tooling, free programmability of motion and angle, and the ability to access hard-to-reach places.
The advantages of the Jetwave's high-velocity are twofold: its heat transfer makes up for reduced contact time while its pull-off angle and velocity (flow rate) make it effective in removing bridges. Vector speed, Z-height control, solder contact time and solder pressure applied to the board are the most critical parameters for determining an application's correct process. Similarly, the depth of the board vs. the fixed nozzle affects the contact area's shape and size. For example, a 10 mm wide Jetwave can provide a contact path of approximately 6 to 11 mm, depending on soldering height. In contrast, with a single-point or multi-dip nozzle, the wave's projection above the nozzle itself dictates that board height must be controlled tightly. The added benefit of the projected wave is that the process window remains large and unaffected as the dip depth changes slightly, owing to board warpage.
UtilizationWith the Jetwave, several control parameters are required. The system must be able to control pump speed, board-to-wave height and the angle of the board to the wave accurately as well as motion speed, acceleration, board rotation angle, and motion direction. Absent such control, the process loses its flexibility and many of its advantages.
Figure 1. How two Jetwave nozzles may be configured to solder a connector from the bottom between adjacent components.
Wave projection above the nozzle enables the Jetwave to access areas between obstacles that standard nozzles cannot reach (Figure 1). Because of the tight placements of adjacent connectors and components in contemporary designs, the process would challenge straight-dip nozzles. If a connector has a tendency to bridge under a straight-dip process, its proximity to other connectors and end components would render the dip-and-peel process a challenge as well.
Figure 2. A typical connector showing the "keep-out" areas for running the Jetwave soldering process. While it typically is run against the motion of the board (a), it also may run across (b) to move keep-out requirements to the other side.
Jetwave technology, on the other hand, can be used for virtually any connector or area of solder joints to be joined. While the Jetwave can be conformed in widths from 4 to 20 mm, limitations actually are defined by the size and location of the "keep-out" areas. For example, Jetwave typically is run against the motion of the board but it also is possible to run it across the connector and, therefore, shift the keep-out requirements to the other side of the component (Figure 2). The arrows show the direction of the wave flow in relation to the connector. Note that board direction is opposite to that of the flow. Because the systems using the Jetwave process can tilt and rotate the board, the change of the wave's orientation is a simple programming adjustment. Additionally, if the board angle is increased, it may be possible to lift the Jetwave directly off the connector without requiring the run-off clearance at the end. It also is possible to start the Jetwave so that the tail edge of the contact area is touching the first pins, which then minimizes the lead-in keep-out area.
From a development standpoint, Jetwave technology retains the free programmability of a single-point nozzle in the sense that a specific nozzle is useful for various components on the same board. The ability to freely program a variety of methods permits the user to develop a process having the widest window possible, with such methods being adoptable without the necessity of hard tooling changes.
Limited Space ApplicationsMost applications for Jetwave require high-velocity solder projection in deference to keep-out requirements. However, in some cases, the requirements are for a shorter contact patch. Figure 3 shows a Jetwave with the rear projection plate adjusted so that the wave resembles a "chip" type wave instead of the high-velocity version.
Figure 3. A Jetwave application with the rear projection plate adjusted for shorter contact arrays.
The smaller and squarer contact path is beneficial to those products having short connectors or quantities of solder points together with limited surrounding space. Because the nozzle still can be moved across the connector or area to be soldered, it permits the process to be dictated by the program and not the specifics of nozzle size and shape. For example, if a parallel array of 20 x 2 contacts with a 3 mm pitch were to be soldered by a straight-dip nozzle, the nozzle itself would feature approximately the same dimensions as that of the connector with additional room for wall thickness. If the same product had the same style connector with 40 rows instead of 20, it would require two dips and the process would have to rely on a straight extraction movement regardless of the connector's propensity for bridging.
For its part, under the same conditions, the Jetwave could be programmed to solder a 4 x 2 to a 400 x 2 connector with the same process and with only changing one coordinate in the program. Although the Jetwave is more flexible in the "chip wave" configuration than traditional straight-dip designs, the process windows are tighter and the uses are restricted when compared to the high-velocity configuration.
ConclusionAs an alternative in the area of selective wave soldering, Jetwave technology bridges the gap between difficult processes that require individual joint control and those that demand excessive cycle times. The ability to move across the soldering area permits a process control that cannot be found in single-dip applications. The wave's high velocity and high-vertical projection adds accessibility and opens the process window toward reducing defects. In terms of repeatability, it is on par with all current wave-type systems because it is controlled by pump system accuracy and solder level controls. With the addition of closed-loop, wave-height monitoring, repeatability can exceed that of the more traditional processes.
Jonathan Wol, president, may be contacted at Pillarhouse USA Inc., 635 Touhy Ave., Elk Grove Village, IL 60007; (847) 593-9080; Fax: (847) 593-9084; E-mail: jwol@pillarhouseusa.com.