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ACCEPTABILITY AND QUALITY OF FABRICATED BOARDS
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FIGURE 51.2 Damage done to PCBs during typical leaded and lead-free solder cycles. (Courtesy of Microtek Laboratories.)
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important to note that each rework process usually includes two soldering cycles: one to desolder the affected part, and one to reattach a new part.
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51.6.1 The Effects of Lead-Free Assembly on PCB Acceptability and Quality The soldering process has a significant and detrimental effect on the PCB s reliability and quality. The lead-free assembly process increases both the soldering temperature and the time the PCB is exposed to that temperature. This will accelerate degradation of the base material and can have significant impact on the PCB. Figure 51.2 shows damage related to time and temperature above Tg. It is essential to establish that the PCB was capable of being assembled and was acceptable when it was delivered to the assembly organization, as poor assembly
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techniques can severely compromise the integrity of the PCB. It is almost impossible to look at a PCB after assembly and definitively state that the fault lies entirely with the PCB manufacturer or with the assembly organization. 51.6.2 PCB Issues Created by Lead-Free Assembly
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Copper reduction Lead-free solder, when molten, rapidly dissolves copper into it. One must be very careful when using lead-free solder on copper surfaces without a protective barrier of some type (e.g., nickel); otherwise, the copper conductors may be reduced to a point where they are not adequate for the design. Base material degradation Base materials begin to degrade and decompose during soldering and can reduce the insulating properties of the resin system in the base material. Conductive anodic filament (CAF) As the base material system degrades, the resistance to CAF formation decreases. Delamination In extreme cases, the resin system can lose cohesive or adhesive properties, causing catastrophic failure. Increased expansion causing premature PTH failures Base materials expand when exposed to soldering temperature. This expansion becomes pronounced above the Tg of the resin system in the base material. The increased temperature of lead-free soldering can cause premature failure of the PTHs due to this increased expansion. See Fig. 51.3 for PTH expansion characteristics with temperature. Solderability Lead-free solder does not wet as well to solderable surfaces as leaded solder does, so pad size, solder quantity, and flux type and quantity must be taken into account.
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FIGURE 51.3 Z-axis expansion characteristics of 150 C Tg PCBs versus temperature. (Courtesy of Microtek Laboratories.)
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51.7 NONCONFORMING PCBS AND MATERIAL REVIEW BOARD (MRB) FUNCTION
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Standards and customer documentation establish minimum quality and acceptability requirements for the inspections performed. If the PCB does not meet these requirements, it becomes nonconforming. This does not necessarily mean the PCB is scrapped, although this is often what happens to nonconforming PCBs. What it does mean is that with a generic standard, samples that do not meet the stated requirements must be dispositioned outside
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ACCEPTABILITY AND QUALITY OF FABRICATED BOARDS
the standard. To determine whether a nonconforming PCB is or is not scrap, a material review board (MRB) must be established to evaluate the nonconformance(s). The MRB is usually made up of one or more representatives from the departments of quality, production, and design. The purpose of the materials review board is to effect positive corrective action within a short time to eliminate the cause of recurring discrepancies and prevent recurrence of similar discrepancies. The MRB s responsibilities include:
Reviewing questionable PCBs or materials to determine conformance with acceptability, quality, and design requirements Reviewing nonconforming PCBs for effects on design functionality Authorizing repair or rework of nonconforming PCBs when appropriate Establishing responsibility and/or identifying causes for nonconformance Authorizing the scrap of excessive quantities of materials
When all else fails and the PCBs do not meet the intended end product use, they need to be scrapped.
THE COST OF THE ASSEMBLED PCB
The cost of the assembled PCB continues to increase because of density, complexity, and rising material and component costs. The value added to the PCB can be 10 to 20 times the cost of PCB. If it is found that a nonconforming PCB is the cause of failure, the total cost of the assembly process is likely to be passed on to the PCB manufacturer.
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