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Reflow Methods Using Integrated Preforms
December 31, 1969 |Estimated reading time: 7 minutes
Solder fabrications, or preforms, can be made to fit many unique applications. Preforms are designed to have a particular shape and deliver a specified volume of solder to the joint. This article discusses how integrated preforms are used and, in particular, methods used to reflow them for maximum quality and production.
By Paul A. Socha, Indium Corporation
Preforms can be divided into two subclasses: conventional, where the preforms are punched from solder ribbon; and integrated preforms, which are chemically etched from solder foil. Integrated preforms are units of solder composed of smaller units, connected by thin strands of solder. These strands are designed to be extremely narrow so as to melt quickly at the onset of the reflow process. To ensure complete separation, the strands are half-etched in the middle to create a breaking point that allows for controlled separation. This half-etching process also reduces the solder volume. Smaller units can be shaped like discs, rectangles, squares, washers, or special shapes (Figure 1). The center-to-center spacing of these smaller units is precise, so they will fit the application and allow for the solder to go where it is required. This article addresses integrated preforms composed of washers; however, this information can be applied to other shapes as well.
Figure 1. Solder preforms are available in a wide range of shapes and sizes.
There are definite advantages to using an integrated preform over a conventional one in a soldering operation. Following are some examples of benefits achieved when using integrated preforms in a thru-hole-connector attachment application.
Time savings. Hand-placing solder preforms is very time consuming. Each preform must be handled separately and placed so it is properly aligned with matching component pads. With integrated performs, many washers can be placed simultaneously. Each washer is the same size as the others, and spaced so that once the integrated preform is lined up with the pins at the end of the rows of a connector, it can be inserted easily onto the remaining pins. This process can reduce placement time (Figure 2).
Figure 2. Washers can be placed simultaneously and are spaced so that preforms can be inserted into remaining pins, reducing placement time.
Quality and cost savings. Integrated preforms with connected solder units ensure that only one unit of solder is placed on the pin being soldered. With separate preforms, there is a concern that more than one preform will be placed on a connector pin, causing bridging or causing a pin to be missed, resulting in a failed joint connection.
Unique designs. Design options for integrated preforms are almost limitless. If you can draw the shape, it can be etched to fit the application. Most integrated preform connector designs are solder washers arranged in a specific pattern with equal center-to-center and row-to-row spacing. However, some applications may require a staggered pattern or a unique approach to supply the correct solder volume to the joints. For example, if it is necessary for solder washers to make contact with connector pins, a tongue or V-shaped projection can be designed into the inside diameter of the solder washer so that it makes contact with the pin. As the pin heats up during reflow, the tongue transfers the heat to the surrounding connected solder washers, aiding the reflow process. Another use of this design is to create friction against the pin, allowing integrated preform washers to stay on a connector while it is being inverted for insertion into the PCB.
Simplify difficult soldering jobs. Thru-hole connectors with long pins are a challenge to hand solder. It is difficult to reach pins in the center portion of multiple rows of pins. Integrated preforms eliminate the need to reach these remote areas. The connected washers can be placed on top of the board, under the connector. After reflow, equal volumes of solder are delivered to each of the pins in the connector.
Solder fortification. After solder paste is applied onto the entire PCB and populated with surface mount components, an integrated preform can be applied to thru-hole components requiring additional solder. This process provides additional solder volume. Components constructed with paste and integrated preforms using this method can be sent through the reflow process at the same time. Thru-hole components requiring only the solder volume from the integrated preform, and not solder paste, can use this method as well.
Storage and handling. Integrated preforms typically are processed according to customer specifications for alloy composition and thickness. The application design is then etched onto 5" × 9" sheets, either as a discrete unit of solder clusters or a complete sheet of washers. Each can be cut from the sheet easily. Prior to shipping, these sheets are cleaned, their quality is checked, and they are packaged in argon. Finally, the package is vacuum-sealed to minimize oxidation. Users should store the packages in their original, unopened container in a nitrogen dry box until use. Once the package is opened and sheets removed, remaining sheets should be returned to the nitrogen dry box until needed again. The advantages of using integrated preforms can be nullified if they are not handled properly. As with any preform, integrated preforms should be handled with care using gloves, finger cots, suction pick-up tools, or tweezers to avoid contamination.
Flux is required. Flux is essential to most solder joints. When using integrated preforms, it is important to coat the entire preform with flux, including connecting stands that join the smaller preform (i.e., washers). A paste flux provides an ideal solution due to its ability to adhere and hold the preform to the area during reflow. Paste flux is available in various chemistries, which makes it easy to use in conjunction with a compatible solder paste, whether it is water-soluble, solvent-clean, or no-clean.
Proper placement. Before reflow, the integrated preform must be flat against the part being soldered. For example, when placed onto the connector pins, the preform must be pushed to the bottom of the pins with a mechanical tool. In most cases, the preform can be aligned in the proper position by merely inserting the pins of the connector in the holes of the PCB and letting the connector bottom-out, thereby flattening the preform. The flatness requirement ensures that the preform will separate evenly across its surface during reflow.
Reflow. Now that the preforms are fluxed and aligned properly, the method of reflow used to melt an integrated preform becomes extremely important. The recommended reflow temperature is 20˚-40°C above liquidus. This ensures that all aspects of the component have reached the same temperature, resulting in the even flow of solder preforms.
Uniform heating of the preform is essential. If one area is heated more than others, the solder in that area will begin to melt and flow to the joint being soldered. When adjacent areas of the preform reach an elevated temperature and start to melt, the cohesive forces of the melted solder will draw the unmelted solder to it. Uneven heating will result in an uneven solder deposition, which means that some joints will have too much solder, while other joints will not have enough solder - solder robbing.
The method of reflow used to make the solder joint often is based on the production volume and available reflow equipment.
Just about any method of reflow will melt the solder in an integrated preform. However, trial-and-error and fine-tuning the reflow process is important to ensure that uniform heat will be delivered over the entire preform.
A soldering iron is not recommended because it will only heat a small portion of the integrated preform at once, resulting in solder robbing. A hot plate can be used for small production jobs, but only if the part being soldered is preheated uniformly.
Hot air guns using nitrogen also are used with integrated preforms in small production jobs to reflow the solder successfully. It is important that the hot nitrogen be blown uniformly on the entire area where the integrated preform is located to ensure that the individual units in the preform become liquidus at the same time. This will result in an evenly distributed volume of solder in each joint. The same holds true with convection reflow ovens for large production volumes.
Companies that reflow integrated preforms using vapor-phase equipment tend to have the highest production yields and the least amount of problems. This is due to the unit’s heat transfer. An inert atmosphere limits the amount of oxidation during reflow, and uniform part heating is maintained throughout this cycle. This reflow method is consistent, repeatable, and ideal for integrated preforms.
Conclusion
Integrated preforms can be custom-made to fit a wide variety of solder applications. Integrated preforms will not perform to the customer’s expectations if they are not handled and processed properly.
Paul A. Socha, Principal Engineer, Indium Corporation, may be contacted at (315) 853-4900; psocha@indium.com.
Technical Support
- Make it a practice to consult with technical support engineers at your solder supplier. Their advice in the early stages of a program is essential to put manufacturers on the right track.
- Technical support engineers can assist with the proper choice of alloys that are compatible with your metallizations, and can offer advice regarding the correct flux to use.
- Preform size, volume, and thickness should be discussed, as should proper reflow methods. Paying attention to these factors will result in a solder unit that will deliver consistent quality, performance, and finished good reliability.