Step 6: Lead forming
06/01/2000
By Michael Dennis
Since the advent of surface mount technology (SMT), chip designers have benefited from the doubling of the amount of available board real estate and ever-developing means of assembly and packaging. At the root of SMT is the selection of the proper lead frame and its associated production process. Materials and operations costs can hinge on this decision, and production tolerances are made or broken based on how well each element of the forming process is planned. Significant technological leaps have been made in the past decade in the field of lead frame design, and it's an area that deserves the design engineer's attention to maximize production yield and throughput.
Lead Frame Selection
Careful selection of a lead frame helps simplify the forming process later in production. It can also have long-term repercussions; witness the historical progression from single in-line packages (SIPs) to dual in-line packages (DIPs) to SMT packages. A single lead can deliver each of the mounting styles and even allow the adoption of some new package not yet invented.
A critical factor in lead selection in terms of the forming process is the tail length, as it must allow enough material to make the required form (Figures 1 and 2). A tail that is too short is useless, but a tail that is longer than necessary can be an asset later. At worst, a long tail increases material costs and generates a bit more scrap (high-quality scrap that can be recycled).
After selecting a minimum length, the head of the lead should be given some scrutiny. The traditional clip-style lead is a sturdy and well-proven design. It can carry a solder slug and even flux to the required location (Figure 3). Offsetting these virtues, it takes up a lot of packaging space both vertically and horizontally, and it needs to wrap around the edge of the substrate, contacting it in at least two places (Figure 4).
Flat leads start to shine in low-profile packages, since they have minimal height requirements. They contact the substrate in only one place and can be clad to carry enough solder for a reliable, non-bridging joint. There are some processing difficulties, but they offer an advantage for custom forming after lead attach, since they are essentially featureless.
The last point to consider in lead frame selection is what kind of carrier may be desired. Most leads come on a single carrier strip, which aligns the leads, maintains their spacing or pitch (Figure 5), and provides a means of indexing the leads. Custom carriers may be needed for small substrates or for processes that demand extremely high precision. Typically, a custom carrier is matched to the process. An example of this would be a 16-pin package that allows multiple substrates to be loaded, then over-molded in strips that match the mold cavities.
The Lead-forming Process
Lead forming is typically done in a die (Figure 6). A relatively simple tool, the die locates the package, clamps it, bends the leads and usually cuts the completed package from its carrier. The locating device may be a nest customized to match the outline of the package, or it may be a simple guide to align the package with the trim and form tooling. A clamp of some sort closes over the leads, holding them in alignment during subsequent operations. A forming tool is pressed against the leads, bending them to the desired shape, and a cutoff tool trims the bent leads to length.
 Figure 2. Shape of the portion of the lead that is used to mount the discrete device to a board. |
For a single-sided package, one set of tooling is all that is required. For two- and four-sided packages, either multiple tools are used or a rotating device brings each side into the tooling for processing. Sometimes the rotating mechanism is actually an operator.
While the tools may seem simple, there are subtleties that affect the success of the process. The first concern is in locating the package relative to the form and cut tooling. For maximum accuracy, the locating device must maintain small tolerances. For maximum yield, the locator must accept variations in the molding or encapsulating process. The ability to juggle these contradictory restraints is crucial to a product's success.
The clamp plays a critical role in maintaining coplanarity of the finished package: If the leads are not held in the same plane during the forming operation, they will not be on the same plane afterward. The clamping must also be repeatable, since variation from one side to the next will disturb coplanarity. The method of lead attach may also complicate the clamp design -there must be enough space for the tool between the package body and the first bend of the form. A traditional clip might, in some cases, make the body of the package too large for the desired footprint after allowance is made for the clamping tool.
The lead-forming process also requires a level of artisanship on the part of the operator. Although there are formulae to predict the behavior of metal during the forming operation, the ideal form of a tool frequently needs to be adjusted by a skilled tool-and-die builder. Metal hardness, thickness variations, plating and lead attach method can all cause the desired form to deviate from ideal. Complicated forms or forms with small tolerances are likely to need this kind of expert tinkering.
 Figure 5. Distance between each lead on a carrier strip or between each pad on a substrate. |
The main issue at the cutoff station is whether the part is cut before or after forming. Some systems cut the leads free first, on the theory that the ends may then move individually during the forming operation. This supposedly delivers a more precise form. The opposite technique, form-then-cut, tends to average out individual form variations. Its advantage is that leads are less likely to get twisted out of progression during the forming process.
Lead forming is amenable to any level of automation. A single-sided hand tool is probably only appropriate for small lots, perhaps in development work or custom packaging. But the basic forming die may be mounted in a press or integrated with other processes. The tools may also be distributed across several dies, forming a progressive process. The process may then be built into a single machine or distributed over several workstations. This can simplify development and enhance flexibility. The downside is that moving from die to die increases alignment difficulties, as each die introduces a certain amount of error; the cumulative result may exceed the tolerance of the package.
 Figure 6. Lead-forming die. |
An example: Part A and Part B have identical stand-off dimensions; in fact, they are the same product except that Part B has an extended "footprint" that allows it to fit the outline of an obsolete product. Both parts may be produced in the same forming die. Then the product stream is split and Part B goes to a workstation set up to cut the leads at the revised dimension. Part A is processed on its own cutoff tool.
Selecting the Right Vendor
Assuming that a company does not have a captive tool and die shop, the selection of a vendor can make or break a process. Ideally, the lead frame, its attachment machinery, and the soldering, cleaning and molding are all supplied by the company that makes the forming die. This way, all tolerance issues may be dealt with at one source. In actuality, few if any vendors can supply an end-to-end process. But it is wise to strive to find the majority of key components from one source and insist on their involvement in the integration process. A forming die will last 10+ years, but may take six months to design and build. Typical process integration may consume as many as 12 to 18 months.
 Figure 7. SEM view of a soldered "J" clip. |
A quality vendor should be able to guarantee that their stampings work with their lead attach and forming tools within the desired levels of precision. They may also be able to tweak parts of the system to compensate for weaknesses, such as adding a bit more plating to help get a better solder joint. And overall, the package cost should offer the best price/performance ratio, since the vendor ought to be an expert at maximizing the yields from their own products.
Ensuring Precision
As with all chip- and board-level production processes, maintaining precise tolerances is paramount. In the field of lead frame connectors, achieving this prescribed goal comes down to selecting the right lead frame, adopting the best production process and enlisting the most capable production vendor. Although it's a relatively simple element of microelectronic construction, the entire production process is literally riding on lead-frame selection. Approaching the specification of the right lead frame while keeping the above points in mind allows for greater production accuracy while maximizing throughput and minimizing cost.
MICHAEL DENNIS, director of research and development, can be contacted at Die-Tech Inc., 295 Sipe Road, York Haven, PA 17370; 717-938-6771; Fax: 717-938-6099; E-mail: [email protected].