NTS News Center

Latest News in Testing, Inspection and Certification

NTS News Center - Latest News in Testing, Inspection and Certification

Using functional test on a PCBA after thermal shock to check for PCB failures

Thermal Testing (without humidity) is typically broken into 2 categories (Shock and Cycling). Thermal Shock is performed using a 2 chamber system that rapidly moves product between Hot and Cold Temperatures. In this test the cold temperatures typically range from -40C to –55C and the hot temperatures typically range from 125C to 150C. Thermal Cycling is a single chamber test that gradually moves product from one extreme to the other at 5C to 15C per minute. In this test the cold temperatures typically range from -25C to –40C and the hot temperatures typically range from 65C to 100C. Both tests have the effect of causing expansion of the product and accelerating failures caused by expansion.

Bare PCB’s are typically tested with optimized daisy chain coupons using thermal shock, as the rate of heat change in the chamber has little to do with the failure mechanisms found in PCBs and the faster transitions significantly shorten cycle times. The extended temperatures typically associated with Thermal Shock also provide an additional acceleration factor which can identify potential issues with fewer cycles. PCBs are usually tested for resistance in the chamber at the high temperature where expansion is at its peak.

Assembled PCB’s are typically tested using Thermal Cycling as the rapid transitions associated with thermal shock can cause temperature differentials between the PCB substrate and the attached chips putting strain on the interconnecting solder joints that can lead to premature joint failure that would not happen in real life. In addition, many components are not rated to perform at the high temperatures typically associated with Thermal Shock and can be damaged when exposed to temperatures beyond their rating. In many cases the assembled board is powered and tested or cycled through its operating parameters during the thermal Cycling.

You described to me a situation where your customer is using functional test of assembled PCBs after Thermal Shock to determine failures in the PCB (cracks and IP Separation). There are several issues about this approach that I would like to discuss. Most PCB’s undergo assembly simulation prior to Thermal Shock so the effect of the process of assembling components onto the PCBA in question is consistent with what would typically done to a bare PCB facing Thermal Shock testing. The rapid temperature change associated with Thermal Shock can have a detrimental effect on the solder joints of the PCBA due to the fact that the component and PCB absorb or give off heat at different rates. This can result in a CTE mismatch between the component and the PCB that could cause solder joint failure. This effect is typically limited to surface mount components. It is also possible that certain components can be damaged by the temperatures associated with Thermal Shock. In some components it is possible that those temperatures would change electrical properties to a point where they would not function properly. These factors make it a imperative when using this technique to verify and track down the cause each failure seen in functional test. This verification is necessary to ascertain whether the issue is PCB, Solder Joint or Component related failure as any of these would cause a functional test failure.

RoHS Recast: Is My Product Affected?

Does RoHS 2 Apply to my product?RoHS, an acronym for Restriction of Hazardous Substances, is a directive  originated in the European Union and was first brought into force in July of 2006 (RoHS 2 FAQ 28). The objective of this program is to regulate the concentration of six hazardous chemicals contained in electrical and electronic equipment (EEE). The substances included are Lead, Mercury, Cadmium, Hexavalent Chromium, Polybrominated Biphenyls (PBBs), and Polybrominated Diphenyl Ethers (PBDEs). Cadmium may not exceed a concentration of 0.01% by weight in any homogenous material, while all others may not exceed 0.1% by weight (DIRECTIVE 2011/65/EU 100).

RoHS Compliance Logos

The Birth of RoHS 2

On January 2, 2013, the original RoHS directive was repealed and has been replaced by the RoHS recast, known as RoHS 2. RoHS 2 is an updated version of the first directive and is mainly aimed at preventing the risks these hazardous substances pose to “human health and the environment, with a particular focus on workers involved in the management of electronic waste,” which is greatly increasing in volume in the European Union. It has been determined that the best way to address these concerns is to control the use of hazardous substances during the manufacturing process (RoHS 2 FAQ 5-6).

RoHS vs. RoHS 2

There are a few important differences between the first RoHS and RoHS 2. Among those differences is a gradual extension of RoHS requirements to all EEE by July 22, 2019, including all cables and spare parts (with some exclusions). Starting on July 22, 2014, Category 8, Medical Devices, and Category 9, Monitoring and Control Instruments, two new product categories that were previously exempt, will be introduced. Another significant difference between RoHS and RoHS 2 affects product marking requirements. Previously under the first RoHS directive, manufacturers would use the following markings, among others, to demonstrate compliance of their product:

Now, under RoHS 2, all EEE within scope must be CE marked. The CE mark, which stands for European Conformity, symbolizes that the responsible party has completed all appropriate compliance procedures for that product. As of January 2, 2013 the CE mark is the only mark that can be used to indicate RoHS 2 compliance (RoHS 2 FAQ 5-6).

RoHS Compliance Requirements

Does RoHS 2 apply to my product?

All of that being said, the question still lingers: “Does RoHS 2 apply to my product?” To decide, you must first determine if your product is EEE as defined in Article 3(1) and Article 3(2). Per the directive, EEE is “all equipment that has at least one intended function which is dependent on electric current or electromagnetic fields, or that generates or transfers or measures such currents and fields…” (RoHS 2 FAQ 19-20).

You may be wondering, are circuit boards considered EEE? “This depends on whether the board is placed on the market as a finished EEE product…, or it is placed on the market as a component for further production or integration in to a finished EEE product.” If the latter is true, then RoHS 2 provisions are not applicable. However, any item sold for direct use by the consumer is considered EEE. Empty, or bare, circuit boards with no components are not deemed finished EEE (RoHS 2 FAQ 18).

If your product does not meet the definition of EEE, then the provisions of RoHS 2 do not apply.

Wait! Your product may be exempt.

If your product does fall within the classification of EEE, there are additional determinations to be made. You must now ask yourself if your product falls within any of the categories excluded by RoHS 2 as specified in Article 2(4). Some of these exempt categories include military/security equipment (e.g., missiles), equipment designed to be sent into space (e.g., satellites), means of transport (e.g., cars, commercial aircraft, trains, boats), active implantable medical devices (e.g., pacemakers), photovoltaic panels, and R&D equipment. If your product falls in one of these categories, then the provisions of RoHS 2 do not apply (RoHS 2 FAQ 16-17).

If your product cannot be classified into an exempt category as listed in Article 2(4), you must now decide if any substance exemptions can be applied. In some cases, an exemption can be granted and the specific use of a hazardous substance can be justified based on certain criteria. Under the first RoHS initiative, exemptions could be granted based on overall practicability. Meaning if the result of using the substitute would be more negative to environmental and consumer safety than the use of the restricted substance, an exemption could be warranted. In addition to these parameters, RoHS 2 has also taken into account the availability and attainability of substitutes, as well as the socio-economic impact of substitution. When an exemption has been applied for, it is decided on a case-by-case basis. “Exemptions are granted for specific substances used in specific applications and not for the whole EEE, nor for a company. Therefore, whoever uses the substances in the specific application can benefit from the exemption” (RoHS 2 FAQ 25-26). As the export.gov webpage entitled, “Restriction of Certain Hazardous Substances” brings out, “these exemptions are temporary and reviewed at least every four years.”

If substance exemptions apply for your product, all provisions of RoHS 2 apply with the exception of those exempt substances. If no exemptions apply, then all provisions of RoHS 2 are applicable.

RoHS applies to my product. Now what?

So, you have determined that RoHS 2 applies to your product. Now what? There are still some important points to keep in mind before you begin testing your product for RoHS 2 compliance. If your EEE product consists of different components, it “can only meet the substance requirements if all components and parts meet the substance restriction requirements of RoHS 2, including non-electronic or non-electric components” such as bolts, wires, or plastic cases (RoHS 2 FAQ 25-26).

Each material in your product must be separated into homogenous samples and tested individually to determine full compliance. “A homogenous material is either a material with a uniform composition throughout, or a material that consists of a combination of materials” that cannot be mechanically separated in any way. In addition, it is also important to know that RoHS 2 restrictions do not apply in the manufacturing process; therefore, restricted substances can be used during production, as long as the finished EEE product does not exceed the set maximum contamination limits (RoHS 2 FAQ 27-28).

If your product currently contains restricted substances and your company needs to transition into RoHS 2 compliance, it may be necessary to perform other types of testing during this process. This will help to ensure that your product maintains the same level of quality and performance.

If your product is destined for sale in the European market, RoHS 2 should be an integral part of your quality program moving into the future. By July 23, 2019, unless your product falls into an exempt category, RoHS 2 regulations will directly affect every electrical and electronic equipment manufacturer (RoHS 2 FAQ 9). For that reason, it is encouraged to begin testing your product, and all its components, for RoHS 2 compliance as soon as possible.