Rework

The decision to rework electronic devices or assemblies depends on the cost of rewoiking versus the value of the part. Consumer electronics, due to their low cost, are considered throwaways and seldom reworked. For some, such as plastic-encapsulated microcircuits (PEMs) that are produced in high volumes, rework is impractical because of the difficulty in removing the plastic encapsulant without destroying the components. However, for high-value assemblies, such as densely populated PWBs and MCMs, where 

Rework generally consists of removal and replacement of a failed electronic componentthat has been detected after initial electrical testing or testing after bum-in. The actual rework procedure is more of an art than a science. Since each component or assembly is different, considerable experience and care are required to selectively remove one device without damaging adjacent ones. 

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Install enough intermediate storage to allow reworking of off-specification material. 

Allowing enough intermediate storage to rework ofT-specification material. 

Section 2 on General Information, Conversion Tables, and Mathematics has had the table on general conversion factors thoroughly reworked. Similarly the material on Statistics in Chemical Analysis has had its contents more than doubled. 

The propellant is dry-screened to remove dust and foreign material, and excess graphite. Unacceptable propellant from the dry-screen operation may be returned to the graining stage for reworking. The product is blended in large rotating barrels for baUistic uniformity. 

Engineering rework is possible with eutectic and solder materials, but impossible with silver—glass. This constraint severely limits the usefulness of the material. Tables 4 and 5 give the electrical, mechanical, and thermal properties for various adhesives. 

Epoxies provide excellent chemical, moisture, and abrasion resistance. However, because a solvent can sometimes dissolve the epoxy present in the laminate, rework on an epoxy coating requires heat or abrasion to remove the coating. 

Quahty control iaspection is a post-, or at best concurrent, manufacture activity. The QC laboratory reports on ia-process and finished product quahty based on testing. Thus QC confirms whether a material has been manufactured in conformance. Sometimes QC also assists the production unit in salvaging process material or reworking off-standard finished product. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Change in feed composition. This may happen due to change in suppliers or due to introduction of reworked material. Unwanted effect on reaction products, by-products. Varying inhibitor concentrations in monomers from different vendors. Potential for runaway reaction. 

Need for reworking, blending, or disposing of partially reacted or off-spec batches. 

Invoices statement of procedure for submitting invoices Quality Control explanation of responsibilities for analysis of supplied materials, finished product, and work in progress consequences of off-spec materials procedure for rework Process Ownership specification of who owns the process, degree of liability for remediation, procedure for review and approval of process changes... 

An example of procedures found by performing a job task analysis would be rework procedures. Occasionally, out of specification product is produced which can be reworked in the process. This subcategoiy of normal operations phase procedures may require special consideration. While rework can be a straightforward operation, it could have safety consequences if done improperly. 

Some tolled products can be reworked if they fail quality assurance tests. In these cases the production cost may go up, but the product is not a complete loss. However, other tolled products, some pharmaceuticals for example, could be completely ruined by similar processing mishaps. Losses in these cases can be quite extensive. Test runs and approved rework procedures for such products may help avoid these losses. 

Are standard production batches with added rework material sampled for quality assurance approval after rework is added ... 

Does all rework of off-spec materials require documented Quality Assurance authorization ... 

This is an extremely important aspect of the whole exercise. The basic purpose of all checks and controls is to reduce reworking, reprocessing, failures or rejections during the work process with a view to produce a product of the required quality and hence minimize cost for better financial returns. A high-cost input not commensurate with the type of the product may defeat the basic puipose of adopting such a system. 

Improperly set tolerances and uncontrolled variation are one of the greatest causes of defects, scrap, rework, warranty returns, increased product development cycle time, work flow disruption and the need for inspection (Gerth and Hancock, 1995). If manufacturing processes did not exhibit variation, quality problems would not arise, therefore reducing the effects of variability at the design stage, in a cost-effective way, improves product quality (Bergman, 1992 Kehoe, 1996). 

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