Standardizing lead scan tolerances: It can and should be done
04/01/2000
Standardizing lead scan tolerances: It can and should be done
JIM STOTTS
It goes without saying that standardization is beneficial - a concept that the industry and individual companies should strive toward. While wafer fabrication, probe and even final component test have been achieving this, there seems to be a lack of standardization for back-end component finishing processes - particularly lead scan. This is a problem that presents itself most clearly to professionals who are working in contract scanning and tape and reel/scan services.
A contractor can see the same type of package (e.g., a QFP-44) from many different semiconductor manufacturers, packages that have been assembled at the same subcontract assembly site using the same Joint Electronic Devices Engineering Council (JEDEC) drawing, but still have varying final scan tolerances (Figure 2). Scan tolerances can be a reference to a JEDEC specification or an assembly specification. These tolerances are measured to determine if a part is within specification for its mechanical dimensions. This is critical when assembling circuit boards, as parts that are out of specification will not make contact with the board, causing test or field failures - both of which are costly.
Causes of Variances
Why do these tolerances vary? First, different test processes and test equipment can damage parts to varying degrees. Second, historical end-user requirements for measurements, such as coplanarity, may cause manufacturers to vary from each other.
Thirdly, some manufacturers may guardband to different specifications based on the scanning equipment set being used. The final and possibly the most likely reason is that there isn`t a complete understanding of existing standards and their practical uses. End users rely on the JEDEC standard, so it should be used when communicating the final tolerances.
Choosing Drawings
JEDEC drawings clearly define the parameters in which a package should be assembled and delivered to an end user. Critical dimensions, such as coplanarity, terminal dimension, standoff, and lead pitch, are defined. Often these dimensions, as defined by JEDEC, are even tighter in the assembly process. That is why it is critical to use the actual case outline drawings from each assembler when profiling a part for use on lead scan equipment; this is how the part was actually assembled and what the vision system on the lead scan equipment will see. The distinction is important because the JEDEC standard is what most end users will hold a manufacturer to, even though actual case outline drawings are still necessary. That is not to say that these two specifications cannot work hand-in-hand. When working with assemblers, semiconductor manufacturers must ensure that both assembly case outline drawings and JEDEC drawings can coexist. If they do not, then the semiconductor manufacturer must work with the assembler to resolve the differences. Then the end user will be assured of the committed tolerances. In the end, parts manufacturers must determine which drawing specifies what the process is capable of and, more important, what the end user is guaranteed to receive.
When scanning parts, the best approach is to use JEDEC as the standard drawing and tolerances that can be guaranteed to the end user. Next, it is necessary to thoroughly understand the equipment set being used to scan the devices. This means that gauge studies for accuracy and repeatability must be performed to validate the accuracy rating of the equipment. Most equipment available today is accurate to ±0.5 mil. This means if the tolerance for a QFP-44 is 4.0 mils for coplanarity (per its JEDEC drawing) the scanning equipment can guarantee that no lead scan escapes at a 3.5-mils reject setting (4.0 mils tolerance for coplanarity minus the machine accuracy guardband of 0.5).
Guardbanding
Guardbanding equipment to its actual capability allows manufacturers to minimize rejecting parts when they are, in fact, good parts. This guardband level should fall in the 0.3 to 0.5 mil range. If an equipment set is not capable of this, then reliability must be questioned. Solutions to this include acquiring more reliable equipment or outsourcing to a company that does tape and reel and lead scanning. A reject setting of 3.0 mils often will be set to accommodate variances in equipment types as a shortcut to properly guardbanding equipment or working with equipment manufacturers to deliver highly reliable systems; this creates wasted costs. It is vital to capture the data of actual yielding, by dimensional reject, to uncover test process or assembly problems. By using this data, manufacturers or assemblers can troubleshoot problems in either test handlers and processes or assembly. This, in turn, increases yield and drives manufacturing costs down.
There are many scanning systems available in the market today. Some are laser-based (Figure 1), while others use cameras to capture lead integrity (Figure 3). All of these systems, regardless of the technology used, must produce accurate, repeatable and reliable results at a minimum of ±0.5 mil. While guardbanding is a tool to ensure quality within a given parameter set, it should not be used to bandage manufacturing problems somewhere else in the process.
Standardization`s Gains
How can standardization be achieved in the scanning process? Lead scanning has evolved in recent years as equipment and scanning systems have improved in accuracy, which allows for better understanding of the capability of assembly and test processes. The goal, however, is to provide consistent and mechanically sound parts to end users, and using JEDEC drawings will assist in this effort. By standardizing based on JEDEC drawings in addition to using assembly drawings and guardbanding to ensure JEDEC compliance, the industry can reduce the variations seen today by both end users and the groups tasked with the lead-scanning process. The actual case outline drawing should be used only to profile the part in the original setup for scanning equipment and related tolerances. Too often, both the JEDEC drawings and assembly case outline drawings are interchanged as the reference for creating a final tolerance set. The real opportunity here is for semiconductor manufacturers and those parties responsible for lead scanning to be consistent in the use of drawings and tolerances and use one set, preferably JEDEC, as the measure of what end users can expect.
JIM STOTTS, vice president of sales and marketing, can be contacted at onQ Technology, Building 4, 8201 East Riverside Drive, Austin, TX 78744; 512-386-8800; Fax: 512-386-8930; E-mail: [email protected].
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Figure 1. Laser-based lead scan/vision system (BGA capable).
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Figure 3. Camera-based lead scan/vision system.