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ACCEPTABILITY AND QUALITY OF FABRICATED BOARDS
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FIGURE 51.15
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Wetted solder joint. (Courtesy of IPC.)
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on the solderability of those portions of the PCB intended to be soldered. Test coupons or PCBs are subjected to accelerated aging for a length of time defined by a coating durability requirement reflected on the procurement documentation, followed by solderability testing. Solderability is acceptable when the surfaces to be soldered have fully wetted. See Fig. 51.15 for acceptable criteria for PTHs, and Fig. 51.16 for examples of non-wetting.
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FIGURE 51.16
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Non- wetted solder joint. (Courtesy of IPC.)
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51.11.1.9.2 Coating Durability Requirements. classified into three categories:
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The coating durability requirements are
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Category 1 Minimum coating durability is required. The PCB is intended to be soldered within 30 days from time of manufacture and likely to experience minimum thermal exposure. Category 2 Average coating durability is required. This classification is intended for PCBs likely to experience storage of up to 6 months from the time of manufacture and moderate thermal or solder exposure. Category 3 Maximum coating durability is required.This classification is intended for PCBs that are likely to experience long storage (over 6 months from time of manufacture) and that may experience severe thermal or solder processing steps, etc. It should be recognized that there may be significant difficulty involved, a cost premium, or delivery delay associated with PCBs specified to meet this durability level.
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51.11.1.9.3 Surface Finishes. Tin-lead solder alloys (e.g., 63/37) are the most popular alloy used for surface finishes on PCBs. Other surface finishes are rapidly finding their way onto the PCBs. IPC-6012 lists more than 20 different surface finishes that are now in use for PCBs. Lead-free alloys are also appearing on PCBs. It is important to understand that the composition and type of the surface finish influence solderability. The procurement documentation must state specifications for surface finish. Methods available for analyzing the alloy composition on the plated PCB include wet analysis, atomic absorption, and x-ray fluorescence (XRF). XRF is popular because of the ease of obtaining the alloy composition and thickness nondestructively. 51.11.1.10 Electrical Inspection. These inspections are performed to verify circuit integrity after processing and substantiate that the electrical continuity, signal integrity, and isolation characteristics of the PCB meet the design specifications. Electrical inspection methods are typically performed on the production PCBs, but evaluation of signal integrity is routinely performed on specially designed test coupons. Two popular nondestructive electrical tests are insulation resistance and circuit continuity tests.These are usually performed on 100 percent of complex PCBs, especially multilayer ones.Automated electrical test equipment with either stationary or moving test probes allows the probes to make contact with the all parts of the circuit pattern in order to verify the continuity and isolation characteristics of the PCB. 51.11.1.10.1 Circuit Continuity. Continuity tests are performed on PCBs to verify that the printed circuit pattern is interconnected as designed. Testing can be performed with an inexpensive multimeter for simple PCBs or with more elaborate automated equipment such as a computer-enhanced bed-of-nails or flying probe tester. These types of automated testers can either be preprogrammed with the circuit interconnection information derived from the computer design files or can use a known good golden PCB as a test template to learn the interconnection information. The preferred method is to perform the continuity test on all PCBs submitted for acceptance. This method is especially recommended for multilayer circuitry, where the internal patterns and interconnections cannot be inspected visually after fabrication. 51.11.1.10.2 Insulation Resistance (Circuit Shorts). The purpose of this test is to measure the resistance offered by the insulation system of a PCB to an applied voltage. Low insulation resistances can prevent proper operation of circuits by permitting the flow of leakage current. This test can also reveal the presence of contaminants from processing residues. The test is performed by applying voltage to isolated nets and measuring the current flow between these nets. Test voltages of 40 to 500 VDC and minimum insulation resistances of 100 to 500 meg-ohm are popular. As with continuity testing, insulation resistance testing typically employs automated test equipment using stationary or moving probes. 51.11.1.10.3 Impedance. Impedance (see Fig. 51.17) is the amount of resistance a circuit offers alternating current (AC) signals. High frequency AC circuits require that the impedance of PCB circuits be matched to that of the components in order to transfer the most signal between the components on the assembly. Proper impedance matching also reduces radio wave emissions (EMF) from the assembly. Impedance is measured using Time Domain Reflectometry (TDR) and is typically measured using a specially designed test circuit on a coupon. Requirements for impedance as well as test circuit design must be supplied to the PCB manufacturer. Variations in impedance of up to 10 percent around the nominal value are expected and are not unusual do to standard process variations in circuit geometry and insulation make up. Tighter control of impedance is possible, but special attention must be made to processes and materials to achieve reductions in variation.
Internally Observable Imperfections These are imperfections where evaluation from the outside of the PCB is not possible. They require the preparation of a microsection in order to view and evaluate conformance. Preparing a microsection of the PCB requires a great deal of skill. The method of sample removal, the microsection materials used, and the grinding/polishing techniques all significantly affect
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