Click on the advert above to visit the company web site

Product category: Compliance Engineering
News Release from: E-Certa | Subject: Component Pb conversion
Edited by the Electronicstalk Editorial Team on 17 August 2007

Preconditioning for component Pb
conversion

Joel Deutsch and Michael Baker explain how proper preconditioning allows older ICs and semiconductor devices to be converted to become RoHS compliant.

Preconditioning is the term for reworking of individual electronic components before assembly in an application This term can be used to include processes like preforming, crimping, trimming or re-tinning

However, in this article it is used specifically to refer to re-tinning procedures to change the solder type on the termination leads.

So can leaded ICs be converted to be RoHS compliant? Yes.

We're talking about a physical process to convert the Pb status of semiconductors.

This process uses widely accepted practices to replace solder on components, but adds some key steps to ensure RoHS compliance when converting to Pb free.

As the solder found on the termination finish of most semiconductors is the only area of concern regarding RoHS compliance, this method can be used to successfully convert a component from containing Pb and failing RoHS to being Pb free and RoHS compliant.

Although this process can be used as a viable solution to supply chain issues, there are some important factors to consider in order to optimise the process for RoHS compliant conversion, reliability, cost effectiveness and other component integrity issues.

The practice of preconditioning components for tin whisker mitigation has been around since the 1950s.

Military/aerospace, medical and others who need exceptionally high reliability rates in components have used preconditioning when necessary in order to achieve their application requirements.

It is our position that this same preconditioning process, with some vitally important adjustments, can also be used to successfully re-tin components to a Pb free solder type and reduce the risk of tin whiskering at the same time.

Pb conversion for RoHS compliance is being used by many in the industry to solve supply chain issues and leftover inventory problems.

With this new process, the possibility to convert existing inventory to Pb free for use in current, RoHS regulated applications has become a viable option.

OEMs can take advantage of this process to save dollars when sourcing parts and also to reuse inventory that was made obsolete by RoHS regulations.

Let's take a look at a study by Technology Forecasters to see who is already using conversion to Pb free to their advantage.

Founded in 1987, TFI is a leading research and consulting firm specialising in global outsourcing for the electronics manufacturing industry and on profitable environmental compliance and leadership.

Its service is used by companies like: Microsoft, Jabil, Cisco, Altera, E-Certa and many more.

According to TFI, over half of electronic product companies plan to convert components for RoHS compliance.

In the benchmarking study "Gauging environmental preparedness and recommendations for best practices" (December 2006), approximately half (52%) of the electronic-product companies surveyed are converting or plan to convert the lead (Pb) status of some of their components.

Also in the same study, although most OEMs converting components to Pb free have started with conversions of standard off-the-shelf components (70%), many are also converting end-of-life inventory components (35%) and some high-end components (29%).

Converting parts from Pb to Pb-free can be performed on a wide variety of part types.

However, it should be noted that is more cost effective in some scenarios than in others.

Components that typically can be sourced for less than a few dollars each are sometimes cheaper to buy new than to convert.

These parts can still be candidates for conversion when end of life, allocation or other supply chain problems affect sourcing, but generally can be sourced new for close to the same cost to convert.

On the other end of the scale, many high end components can be converted for significantly less than their cost new.

This can save OEMs substantial dollars while also providing a means to use their existing inventory leftover from pre-RoHS sourcing.

Many OEMs have found conversion to be a great tool to solve budgeting and obsolete inventory issues.

When considering conversion, there are some things that an OEM should look for in a company that converts components to RoHS compliance.

First, what testing is done to ensure RoHS compliance? Is there a step in the process for conversion that removes the restricted elements from the device?.

Secondly, what standards are used to convert the components? Are there widely accepted standards used in the processes at the company? Is there any accreditation or outside certification?.

And thirdly, there need to be sufficient data to verify that the process does not affect the functionality of the components.

Look for results that cover electrical, pull strength and reliability testing.

First, let's look at methods for assuring that the process converts components to RoHS compliant levels.

This first step begins with investigation of the part itself.

Is there a possibility of Pb or other RoHS restricted elements contained anywhere in the component other than the solder on the termination leads?.

A semiconductor package contains many homogeneous materials, which include plastic moulding material, tin-electroplating coatings on the lead frame, the lead frame alloy and the gold-bonding wires.

For many components, knowledge of where the restricted substances are likely to be found simplifies the analysis procedure.

In a semiconductor package for example, the only location where a restricted substance is likely to be present is the lead in the tin plated termination coating.

This can be analysed using ED-XRF (cadmium may be used in the inks used to mark components but this is very unusual).

Next, what methods of testing are done by the preconditioning service provider to ensure that the converted components are RoHS compliant?.

There should be some screening done to verify that the particular batches of parts were converted successfully.

The most common screening method is XRF.

For accurate testing within the 1000ppm limits set forth by RoHS, extra steps need to be taken to ensure that the XRF test is properly calibrated.

The screening lab should have SOPs that ensure repeatable and reliable test results and preferably be accredited by an organisation such as the American Association for Laboratory Accreditation (A2LA).

There should be SOPs that investigate and confirm that parts were converted to the correct solder type and that a removal of restricted elements was done prior to conversion.

This removal of Pb prior to the conversion process is vital as XRF screening may not detect Pb contained in the layer beneath the new solder coating.

A converted part without removal of the original Pb solder may pass XRF, but fail destructive testing for Pb.

Some preconditioning companies do not apply a qualified stripping process, yet still claim RoHS compliance.

Beware of any conversion process that has not had or offers destructive testing performed on parts.

Many are screening their own inventories with handheld XRF devices.

This method is effective for screening large areas of the board for gross violations of the RoHS limits.

In an article by Rob Speigal titled "Testing for compliance heats up", Jeff Shaffer, SVP of Product at Newark InOne says: "Some companies have a gun and they point it at the component and it will say that bromide is present, so they're rejecting the component".

"But the test cannot identify whether it's banned substance".

Some bromides are fine, others are not.

Also, the testing beam emitted by handheld XRF is not small enough to test the solder joints on most IC package types.

As a result, it factors in the area of the board surrounding the IC when calculating elemental content levels.

This can skew the results.

In order to get a more precise reading for an individual component, a smaller test area (collimator) is needed.

OEMs using handheld equipment as a final analysis may be acting on inaccurate results.

A desktop XRF machine can solve this problem because it has a smaller collimator size and the sample to be tested is secured on a fixed platform.

OEMs who are screening their assembled PCB boards for RoHS violations are performing a necessary step of their due diligence obligations.

However, when converting components, it is important to receive test results from the conversion laboratory stating RoHS compliance of the converted component.

These tests should rely on either destructive testing or a desktop XRF machine screening which can pinpoint smaller areas on the components, such as the termination finish, to verify that a proper conversion was performed.

There are also some key factors to ensure that the XRF screening is performed accurately.

Let's take a look at some XRF technical specifications to see what's needed to test for RoHS compliance on converted components.

It is important to have a sample at a fixed distance and for a period of no less than 1min to obtain the 1000ppm level of accuracy needed for RoHS compliance.

Only a properly calibrated desktop XRF instrument can do this which enables it to detect the RoHS elements with the accuracy needed for such low-level detection.

This is not as easy as it sounds, the key being a proper calibration and a specific program that defines the elements searched for.

Contrary to some beliefs, a desktop XRF machine is not as "turnkey" as it may sound when it comes to RoHS testing.

While it is true that it can detect the approximate%age of Pb in a sample on an out of the box standards free calibration, several extra steps are needed to determine all RoHS restricted elements and their precise values.

First, a proper calibration through use of reference foil standards is needed to "fine tune" the instrument to be able to detect with the accuracy required by RoHS limits.

Secondly, you need to tell the machine exactly what it is looking for through use of a program that defines the characteristics of the spectra involved and distinguishes between them.

Then, you must crossreference the machine against test items of known value, preferably obtained through destructive analysis.

Finally it is important to perform a matrix of tests to determine that the levels detected are repeatable and accurate.

Only with these criteria as well as an XRF technician with proper training and experience with spectrometry related to RoHS, can one achieve the desired level of accuracy needed to determine RoHS compliance.

After an investigation is done to assure that a company is converting parts to be truly RoHS compliant, a look at its standard operating procedures can identify any possible problems with the conversion.

There should be all applicable JEDEC (or others) standards in place and followed during the conversion process and for testing of the conversion.

With Pb-free conversion, some extra details about the process should be considered.

Typical solder pot maintenance schedules should be intensified to prevent Pb contamination.

Testing of the solder pot purity should be done at smaller intervals as well as changing of the solder pot.

A reworking for Pb-free standard should include specification limits and guidelines for testing solder for Pb contamination.

Furthermore, it is recommended that solder be changed at least every 15 days.

For Pb-free rework, this may need to be studied and possibly reduced to take into account possible Pb contamination as well as other physical characteristics related to Sn.

It is vital that a qualified stripping procedure be taken prior to the new solder dip in order to guarantee RoHS compliance.

If a series of Pb components are dipped in a Pb-free solder bath with no prior removal of the termination finish, two issues arise that could sacrifice RoHS compliance of the part.

First, without a stripping process, the original electroplating finish is simply overcoated with RoHS compliant solder.

An although this can pass XRF screening, it may fail destructive testing due to violating amounts of the original solder left on the part.

Secondly, after certain amounts of dipping, the solder pot itself runs a risk of becoming contaminated with Pb in amounts that violate RoHS limits.

There are two currently two methods for re-tinning - robotic and nonrobotic.

Both are viable methods to ensure RoHS compliance, but there are some differences in their requirements.

Robotic re-tinning: uses a vacuum, magnetic or mechanical pickup mechanism; is not damaging to the component or its terminations; does not use metal surfaces that may scratch or cause solder smears; must be capable of controlling the dwell time in the solder pot within +/-0.1s; must Be capable of controlling the exit speed out of the solder pot to within +/-0.3cm/s; must be capable of controlling the depth of immersion to within +/-0.1mm; and must ensure dipping of the full lead length into the solder pot including the side of the package.

Nonrobotic re-tinning: must not damage the component or its terminations; must not impede the solder coverage of the component termination(s); must not use metal surfaces that may scratch or cause solder smears; must be capable of immersion of conductors not to exceed 5s maximum; must be capable of controlling the immersion/emersion rates, dwell time and immersion depth; must ensure dipping of the full lead length into the solder pot including the side of the package; must ensure perpendicularity of through-hole leaded components to the surface of the solder; must immerse leaded surface mount components at between 20 and 45 degrees to the solder surface; and should minimise wobble, vibration and other extraneous movements.

After the industry shift to Pb free, the ability to precondition components to a Pb-free state and retain properties that reduce tin whiskers, can solve a myriad of supply chain problems.

It is the best known solution for reuse of existing Pb inventory, and could be a promising solution for green and reliable products.

However, there must be some standard to ensure that converted components are being re-tinned after removal of RoHS violating substances.

Due to the limitations of XRF screening, destructive testing must be done in order to show RoHS compliance of converted components.

It is through this method that it can be determined whether a step to remove Pb was used in a conversion process.

Many manufacturers are in the process of testing or are already using this process to solve supply chain issues.

It is an attractive option to many of them because it provides an outlet to use their leftover inventory in green products.

It can often be a cost effective alternative to sourcing new parts.

It is also a great solution when a particular part is no longer available due to end of life or allocation issues.

Keeping all of these advantages in mind, it still remains important to inspect a conversion company before assuming that their converted components are RoHS compliant due to the number of "overcoaters" in the industry and the possible consequences of accepting these parts as RoHS compliant.

Overcoating without stripping could cause RoHS counterfeits to enter the supply chain.

Overcoating is a technique that is currently being used as a method to mitigate tin whiskering.

Parts that are plated with tinned solder types are dipped in a higher Pb concentration solder where intermetallic bonding takes place.

This creates a pressure stress barrier that hinders the growth propensity of tin whiskering.

However, according to the iNEMI Tin Whisker User Group's report titled "Recommendations on lead-free finishes for components used in high-reliability products", there is evidence that this technique may not be as effective when performed on the new 99% tin RoHS compliant product entering the supply chain today.

Based on the iNEMI report and on the NASA Office of Logic Design paper " Tin whiskers - a 'new' problem" it is Joel Deutsch's opinion at E-Certa that a valid tin whisker mitigation technique should also include a qualified stripping process to the original substrate, which creates intermetallic bonding with copper instead of the original tin/Pb finish.

This practice along with proper annealing and robotic hot solder dipping techniques should be used to form an effective tin whisker mitigation approach.

Using this overcoating/re-tinning technique with another purpose in mind, some claim to convert components to RoHS compliant by simply overcoating the Pb lead with a compliant solder.

However, there are some major issues with this technique concerning RoHS compliance.

First, the Pb is never entirely removed.

The troubling fact here is that these parts in some cases can pass an XRF screening, but are most certainly not compliant.

Though this procedure in small quantities may pass XRF inspection, they will absolutely fail destructive testing, which will be the final method used in any litigation claims against non compliant product.

If destructive testing is ever done for the device, there is substantial risk that Pb could be detected at levels in violation of RoHS limits.

The second problem is that during the dipping process, the intermetallic bonding that is formed adherently still carries trace amounts of the original factory plated coating.

It should be noted that XRF scanning will show failure from larger quantities (500 pieces and above) processed in this false manner due to Pb leeching consistent with repeated hot solder dip procedures.

If you are going to convert a part for RoHS compliance, it is strongly suggested that you use a qualified facility, preferably with licensing of patented procedures that include a stripping process.

They should employ robotic dipping techniques to produce a repeatable effect that assures coplanarity and solderability.

The facility should be capable of adapting in order to use other dipping techniques when circumstances dictate.

Also, they should have SOPs for maintaining solder pot integrity that include regularly scheduled checks and have had their processes tested with an accredited industry laboratory.

Finally, they should have extensive experience and expertise as well.

E-Certa and Sanmina SCI co-operated in a study to test the reliability of converted components.

Different part numbers were used and different types of conversions were performed.

90 parts were provided for each different type of conversion.

A 100% matte tin plated leads were stripped and re-plated with eutectic Sn-Pb finish or a lead-free finish using a robotic process.

The converted components were inspected for damages.

No anomalies were found on the converted components evaluated via XRF analysis, electrical testing and optical inspection.

The quality of the solder joints formed using the converted components were Evaluated for: impact of surface finish; impact of thermal annealing; and pull testing.

The IMC growth for all the converted components (SnPb and Pb-Free) were typical and of acceptable thickness and displaying typical morphology.

Among the PCB finishes, ENIG had the highest mean pull strength.

However, after annealing, it had the lowest mean pull strength.

A similar trend was observed for both SnPb and lead-free parts.

The pull testing results indicate that mechanical strength was not affected significantly during the conversion process.

The "robotic stripping and solder dipping process" can be a very useful process to convert a termination finish to the desired termination finish.

Comments from readers

Gabriele Sala writes: Interesting solution. Those components (ICs: PLCC, QFP, etc) designed to withstand solderability temperature of SnPb components, even if RoHS converted, would they be able to withstand the high temperature solderability required by Lead Free Reflow Profile according to Jedec J-STD-020c? Talking about IR or Forced Convection Reflow and not Vapor Phase Reflow? Any concerns?

• E-Certa: contact details and other news
• Email this article to a colleague
• Register for the free Electronicstalk email newsletter
• Electronicstalk Home Page

Search the Pro-Talk network of sites

Put your news on Electronicstalk  |   Advertise  |   Get the free Electronicstalk newsletter  |   Electronicstalk Home
About Electronicstalk  |   Copyright © 2000-2008 Pro-Talk Ltd, UK