Eliminating time-consuming hand soldering, these linked solutions speed through-hole, surface-mount, and thick board designs.
BY PAUL SOCHA
Integrated preforms are linked solder preforms that allow manufacturers to place a cluster of solder units in position on a board in one quick motion rather than placing the preforms individually in multiple motions. This revolutionary process simplifies the soldering process and improves quality, thereby saving time and money for many manufacturers.
Integrated preforms were developed as a way to streamline the hand soldering operations for manufacturers who were placing tiny washers on connector pins with tweezers, or hand soldering through-hole components on the back side of the board with wire solder. Since their introduction, integrated preforms have been designed for applications ranging from small and simple to large and complex.
Design, alloy versatility
Used in a variety of soldering applications, integrated preforms can be made in countless designs and in more than 200 alloys. Applications include:
- Through-hole component attach–When using integrated preforms on boards without surface-mount components, flux is applied to the integrated preform unit, which is then placed onto the multiple pins of the component and inserted into the board prior to reflow.
- Mixed-technology, surface-mount, and through-hole component attach–In this application, solder paste has already been applied to boards that have been populated with the surface-mount components. To attach the through-hole components, the integrated performs–coated with a paste flux chemically compatible to the flux vehicle in the solder paste–is placed on the components. These components are then inserted into the through-holes, and the entire board is reflowed.
- Thick boards requiring more solder volume–Thick boards pose a challenge with through-hole components because they require more solder than solder paste alone can deliver. In this case, the integrated preform is positioned onto the components, which is then placed on top of the solder paste that has been applied to the through-holes. The flux vehicle in the paste also fluxes the integrated preform so no additional flux is needed. This combination ensures that a sufficient amount of solder is delivered to the joint after reflow.
- Soldering components with long pins–Through-hole components with long pins are challenging to hand solder, especially if they include multiple rows and a fine pitch. The concern is that the inner-most pins will either not get an adequate volume of solder or that some pins will receive too much solder, which could cause bridging. In this application, integrated preforms are used either on the top or bottom side of the connector and, once reflowed, will ensure that every pin has been connected with the same volume of solder.
- Backside soldering solution–Integrated preforms can also be used on the backside of the board before or after the topside has been reflowed. If the paste has been reflowed, flux-coated preforms can be placed on the lead side of the components and reflowed using localized heating. A small hot air gun works well if kept moving so that uniform heat is applied to the entire area of the preform. If integrated preforms are used prior to reflow of the paste, they should be flux-coated, inserted onto the pins of the component, and then positioned against the pads on the backside. Once this is done, the board is ready for reflow of the paste and the preform. To ensure that the integrated preform will stay in place during reflow, a small protrusion (tongue) can be designed into the inside diameter of the washers. Once inserted, the tongues rub against the pins and hold them in place while the board goes through the reflow process. The solder then wicks up the hole to make the connection, allowing for an adequate fillet.
Although these are just a few of the more common applications, the design possibilities are endless and can accommodate almost any solder need.
What is required?
To design an integrated preform, the following information is needed:
- Hole size;
- Pin size;
- Board thickness;
- Alloy;
- Pitch of the pins, both within the row and row-to-row, to determine the outside diameter (OD) of the washer. This is to allow for the proper “necking-down” of the small strands of solder that connect the adjacent washers. If the OD is too close to its neighbor, the strand may be too thick and not separate completely during reflow. If the OD is too far away, the integrated preform will be flimsy and difficult to place over the pins of the components.
Although integrated preforms can be used in a wide variety of applications, they have their limits; however, these are based more on the manufacturing limits of the preforms themselves.
The minimum thickness of a preform is 0.002” and the maximum thickness is 0.018”. If a thicker preform is needed, multiple preforms can be stacked to reach the required amount of solder.
Reflow keys
The reflow method is essential to the success of integrated preforms. A uniform amount of heat is needed to ensure the entire unit is at the same temperature at the time of reflow. When the melting temperature of the alloy is reached, the extremely thin connecting strands will separate first and wick back to the main body of the preform that is close to melting.
 FIGURE 2. The vast majority of integrated preforms are made in the shape of washers, but discs, rectangles, squares, and frames are also available. |
If the temperature is not uniform, portions of the preform will melt first, robbing solder from the adjacent preforms. When this happens, some pins will receive too much solder while others may get very little.
Vapor phase and convection reflow are uniform and work extremely well with integrated preforms, as does a hot air gun for smaller jobs. A hot plate or hand soldering methods yield unreliable results.
Packaging, shelf life
Integrated preforms are manufactured in sheets, aligned to maximize the amount of preform units per sheet. Designs include rows and columns of washers or pads that can be easily cut to the desired shape, while other more complicated designs are tabbed in and can be removed from the sheet by cutting the outer connecting tabs.
The number of units being soldered in one program will help to determine not only the form of the solder, but also the method of placement and packaging.
Multiple sheets of integrated preforms are stack-packed together with rigid packing material and then vacuum-sealed in a bag.
Single sheets can be vacuum-sealed separately for individual use, allowing the other sheets to remain sealed to avoid exposure to the air and oxidation. If units of integrated preforms have been separated from the sheet prior to shipping, they are packaged similar to individual preforms.
Storage, shelf life
Integrated preforms should be stored in a nitrogen dry box when not in use. (This applies to opened as well as unopened packages.)
The shelf life of integrated preforms is dependent on the alloy. Typically, this would be one year from date of manufacture if the amount of lead in the alloy is under 70%, and six months if the lead amount is over 70%; however, this varies from one solder supplier to another.
Integrated preforms, including the connecting strands, should be uniformly fluxed by brush, spray, or dipping procedure. The type of flux is determined by the metalli- zations being soldered and the compatibility of the flux with other fluxes used on the board.
Care should be taken to use similar flux chemistries. In other words, if a no-clean paste is being used, a no-clean flux should also be used with the integrated preform.
Simplicity and consistency
Integrated preforms can be used for many different applications, are easy to use, eliminate tedious hand soldering, and can be integrated easily into existing soldering processes. Most importantly, they can deliver a consistent volume of solder to the joint, which improves the quality and reliability of the finished product. CS
PAUL A. SOCHA is principal engineer with Indium Corp. (www.indium.com)
Advantages of integrated preforms
- Endless designs to fit any application. Integrated preforms can be made in any shape to properly apply the required volume of solder to the joint. The vast majority of integrated preforms are made in the shape of washers, but discs, rectangles, squares, and frames are also available.
- Ease of placement. Preform washers only need to line up on one side of the connector pins. The design will ensure that the rest of the washers are aligned to slide easily onto the remaining pins. When fluxed with a tack flux, integrated performs–designed with pads and other shapes–can be placed and held securely while the component is positioned.
- Elimination of hand soldering. Integrated preforms can be placed and ready for reflow in much less time than it takes to hand solder the individual pins.
- Quality enhancement. Separate washers placed on the pins result in more uniform solder volume than hand-soldering with wire. Integrated preforms make this process worry-free because they eliminate the problem of missing pins or putting multiple washers on the same pin.
- One cleaning. If integrated preforms are used in conjunction with solder paste, only one cleaning process is needed to remove post reflow flux residue from both forms of solder. –P.S.
