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Retinning PCB Re-tinning

Retinning PCB Services Offered

  • Component re-soldering
  • Termination finish change over
    • Lead-free to leaded solder
    • Gold leads to leaded solder
  • Tin whisker mitigation
  • Improved solderability
  • XRF validation
  • Component re-taping
  • BGA re-balling

Got Lead?

Why do I want leaded solder on my component leads?

Ever since the European Union banned the use of lead in most electronics applications, the major semiconductor companies switched from making their components with tin-lead finishes to tin finishes. Many components with lead finishes have quickly become obsolete. The U.S. military and NASA, however, have banned the use of pure-tin in many of their critical applications, because of the tendency for tin to grow metallic “whiskers” that over time, can cause electric shorts.

What is retinning?

retinning is the process of re-soldering component leads to change over the final finish from one metallic type to another. Components of all types may be re-soldered, from capacitors, resistors, connectors, IC’s and BGA’s. Electronic components have various finishes depending on the component type and the manufacturer. Examples include: tin, tin-lead, gold, silver or sometimes they are just labeled as “lead-free” or “RoHS” compliant, without disclosure of the exact metallic content.

What is a tin whisker?

According to NASA:

"Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish. Tin whiskers have been observed to grow to lengths of several millimeters (mm) and in rare instances to lengths in excess of 10 mm. Numerous electronic system failures have been attributed to short circuits caused by tin whiskers that bridge closely-spaced circuit elements maintained at different electrical potentials."

See this NASA website for more details:

http://nepp.nasa.gov/whisker/background/index.htm

How much lead is required to prevent tin whiskers?

Estimates vary, but most experts believe that the component finish only needs to contain at least 3% lead in order to prevent tin-whiskers.

Are there other reasons to do retinning?

Yes. For example, some connectors will be supplied by the manufacturer with gold leads. According to the J-Standard-001, gold leaded connectors must be solder dipped prior to final assembly. Also, a component may need re-soldering if the final finish has oxidized or the solder has otherwise degraded over time.

Do lead-free components on a leaded assembly cause problems?

Yes. The degree of the problem varies according to the component type. The main reason for this problem is the difference in melting temperatures between leaded components and lead-free components. If you setup your reflow oven for a leaded assembly, some of your lead-free components may not properly melt, resulting in “de-wetting” or cold solder joints. If you setup the process for lead-free temperatures, you may exceed the maximum temperature ratings of some of the leaded components. If you cannot find a happy medium, the process could be prone to excessive rework and touch-up in secondary operations. This drives up labor time, turnaround time and ultimately cost.

What are the different melting points of leaded solder and “lead-free” solder?

Most leaded assembly melt between 179 C and 183 C and most “lead-free” assemblies melt between 217 C and 227 C.

What is XRF validation?

X-Ray Fluorescence validation is the process of measuring the metallic content of the components (either before or after) retinning. This allows us to certify that we have met the minimum requirements for lead-content, or certify the absence of lead, for lead-free applications. (For an official definition: see http://en.wikipedia.org/wiki/X-ray_fluorescence)

What size components are typically subject to re-soldering?

All sizes of components can be subject to retinning, however, most applications DO NOT re-tin passive components that are 0402 in package size or less. We believe that at this size level the risk of thermal shock damage, far outweighs any potential benefit for solder-ability or risk from tin whiskers. In addition, retinning passive components can be a very expensive process, considering the high quantities of components on even the simplest circuit board assemblies.

Why is component re-taping important?

If small package size components are to undergo retinning, the most critical step in the process becomes re-taping for automated SMT assembly. Since the retinning process adds solder to the termination finishes, the actual package size may grow enough to make it impossible to re-input the component in its original reel packaging. Custom size tape thus may need to be created. This is a very critical step because the wrong packaging can create a high level of SMT “mis-picks” that drive up overall component attrition. In addition, any components not placed by machine, must be placed in second stage operations, which is very time consuming, labor intensive and subject to error, unless high levels of quality control are implemented.

What is BGA re-balling?

BGA re-balling is the process of removing and replacing solder balls on the underside of the BGA package. Most often, this is done to convert a lead-free BGA to a leaded BGA. The opposite can be done, however, the maximum temperature rating of the BGA must be checked prior to performing this operation otherwise the higher temperature of the lead-free process may damage a leaded BGA.

Are lead-free BGAs on a leaded assembly a problem?

Yes! Lead-free BGAs on a leaded assembly can be a major problem, due to the large temperature mis-match between the 2 style of components. Mixing components is always an issue, however, the problem becomes more acute when they are BGAs because of the lack of clear visual inspection. X-ray inspection and fiber optic cameras help spot these type of problems but they are not 100%. Also, this merely detects the problem and does nothing for prevention. Lead-free BGAs on a leaded assembly can create larger than normal solder ball voids or cold solder joints underneath the package. Defects at this level are thus harder to detect, more difficult to rework and more time consuming to fix, and can cause PWB damage if not handled correctly.

What is a RoHS compliant component?

The European Union banned more than just lead. In order to be RoHS compliant, the components cannot contain any of the 7 banned substances, which are:

  • Lead (Pb)
  • Mercury (Hg)
  • Cadmium (Cd)
  • Hexavalent Chromium (Cr6+)
  • Polybrominated biphenyls (PBB)
  • Polybrominateddiphenyl ether (PBDE)
  • Acrylamide

Thus, there can be a difference between “lead-free” and “RoHS Compliant”. Many RoHS compliant components do not disclose the exact metallic content, thus making the SMT reflow oven profile more challenging. These metal contents can also vary from lot to lot, depending on the facility of manufacture, creating further process engineering and process control challenges for the board assembly house.


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