DVD-71C IPC-A-610E Common Inspection Errors - IPC Training ...

DVD-71C TranscriptGull Wing LeadsAt this point, we’ll take a look at some common misunderstandings associated with surfacemount – gull wing leads. Misidentifying gull wing leads can cause the inspector to apply thewrong rules to the component they are inspecting. The general inspection criteria is dependent onwhether the gull wing lead has a long foot or a short foot. For example, If there is a long footlength – meaning that “L” is equal to or greater than three lead, or toe widths – the side jointlength should be a minimum of three lead widths, or 75% of the foot length, whichever is longer.If there is a short foot length – meaning that “L” is less than three lead, or toe widths, theminimum side joint length is 100% of the foot length.There are also different rules for the low profile leads found on SOICs and SOTs; and the highprofile leads found on QFPs. In addition, heel fillets for gull wing leads provide the mostimportant electrical and mechanical connection. Therefore, care must be taken to make sure thatthe heel fillet is the proper height. The fillet height may extend to the top bend of the lead, orknee, but should not touch the component body or end seal as a maximum fillet height. As anexception to this rule, solder may touch the body of plastic SOIC or SOT components. If youoverlook this exception, you may ask for unnecessary rework – thereby creating extra operatingcosts for your company.Bottom Termination ComponentsNow that we’ve discussed gull wing leads, let’s examine another category of surface mountcomponents called bottom termination components, or BTCs. Misunderstanding these new parts,or the new grouping of many older parts can be a cause for misapplying the 610 standards.Examples of BTCs include Dual Flat No-Leads, or DFNs; Quad Flat No-Leads, or QFNs; andLeadless Chip Carriers, or LCCs. There are other types of leadless components that don’t fallinto the BTC category. For example, array style components, such as ball grid arrays, or BGAs,have their own acceptance requirements.It’s important to apply the dimensional criteria specified in Table 8-15 for Bottom TerminationComponents. Figure 8-170 illustrates the heel and toe dimensions for BTCs. Dimension Cdefines the minimum end joint width. Notice that the minimum end joint width is 50 percent ofthe termination width for class 1 – and 75 percent of the termination width for classes 2 and 3.MeaslingNow, let’s take a look at how measling can be misunderstood. It’s not uncommon for aninspector to examine measling on the circuit board and to identify it as a defect. After all,measles can be pretty ugly. Measling occurs in laminated base material when the glass fibers areseparated from the resin at the weave intersection. This condition appears as discrete white spotsor crosses below the surface of the base material, and is usually related to thermally inducedstress.6