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PCB Designer's Notebook: Improving Industry-developed Standards
December 31, 1969 |Estimated reading time: 4 minutes
By Vern Solberg
There are a number of standards that have been developed within our industry domestically and internationally. Some standards are created as guidance documents, while others specify materials, product requirements, and qualification criteria. Keeping these standards current and up-to-date is a daunting challenge; we often are asked to comply with a document that is several years old.
Component package technology, for example, has expanded rapidly and some such products are not covered in newer documents. Many IPC documents will receive frequent review and updating, especially widely-used ones. J-STD-001 defines the requirements for solder joints on the electronic assembly, while IPC-A-610 furnishes guidance for assessing solder joint acceptability. These two documents have a great deal of industry support and were recently overhauled to include new component technologies. They now contain quality assessment guidelines for lead-free soldering. There are international (IEC and ISO) standards that focus on these topics, but they are based on submitted documents from member countries. Due to the limited expert resources available to participate in the international working groups, these documents are often years behind current technology.
Another popular document undergoing modification and updating is IPC-7095, Design and Assembly Process Implementation for BGAs. Some issues being addressed are related to the physical impact of lead-free soldering. For example, mechanical overstress on the PCB land that exceeds the laminate strength can cause laminate cracking and separation of the PCB land and the BGA from the laminate. This can result in the failure of a joint either mechanically, electrically, or both. The initial crack weakens the joint mechanically; as it progresses, the crack may even intercept electrical traces, causing an open. Higher lead-free reflow temperatures, laminate decomposition, and increased pad or trace densities enhance failure by this signature. As detailed in past articles, the FR-4 circuit board material specification, IPC-4101B, is undergoing extensive expansion to comply with RoHS and meet requirements of lead-free soldering. Initially, laminate suppliers were attempting to develop a single slash sheet (PC-4101/99) to meet the criteria of RoHS and minimize decomposition rates and Z-axis expansion rates during soldering processes exceeding 260°C. Because manufacturers use widely varying compositions to manufacture laminates, a single slash sheet became unworkable. Currently, the IPC is reviewing six RoHS-compliant specifications with slight differences in composition elements with a Tg range between 110°C and 170°C, and a decomposition (Td) range between 310°C and 340°C. All of these have a UL-flammability rating of V-O. Selection of only one variation, however, may force the fabricator to purchase material from a single supplier. The designer, along with a materials expert and the PCB fabricator, may want to include two or three FR-4 variations on the specification sheet to ensure availability from a wider supplier base.
Another phenomenon discovered by one of the task group member companies was the formation of micro-voids during solder processing. Normally, voids within BGA solder ball are not a reliability concern. But in some examples, a massive number of small voids occurred at the ball-to-land interface. The concern is the impact of stresses during thermal excursions that can promote crack propagation at the interface. In some cases, cracks extended across the ball-to-land interface within 500 thermal cycles. The updated document will detail a number of solder-process defects and offer suggestions as for their cause, and possible corrective actions.
In a recent presentation addressing PCB materials during the JISSO International Council (JIC) meeting in Berlin, Germany, it was noted that although RoHS forbids the use of some bromine compounds, it does not ban brominated materials used as flame-retardants for circuit boards. Bromine-containing compounds that are outlawed by RoHS are those that remain as independent molecules within the polymeric matrix. These include polybiphenyl ethers or oxides (PBDE or PBBO) and polybrominated biphenyls (PBB). Bromine-containing compounds that are compliant with RoHS include those that react to become a chemical part of the polymeric matrix, for example, Tetra-Bromo-Bis-Phenol A (TBBPA). Being RoHS-compliant does not mean that the base material must be halogen-free. Certain brominated flame-retardants, including the popular brominated flame-retardants for FR-4 TBBPA are accepted by RoHS; Decabromo Diphenylether has been given a RoHS exemption.
The JIC is an international advisory body created to promote a strategic partnership among organizations interested in the total solution for interconnecting, assembling, packaging, mounting, and integration systems design. To accomplish these objectives, members are encouraged to monitor market trends, develop technology roadmaps, address environmental issues, and support standards development nationally or internationally. The term "JISSO" reflects the total packaging solution for electronic products. As such, this term includes interface and solution technology between users and suppliers for interconnecting, assembling, packaging, mounting, and integration systems design. For more information on the JIC, visit their website.
Vern Solberg, SMT Advisory Board Member, is the senior applications engineer and technical advisor to in-house and OEM engineers and specialists at Tessera Technologies Inc. Additionally, Vern holds several patents for IC packaging innovations and is a member of many industry standards organizations, including IPC, SAC, IEC TC91 and the JISSO International Council. He may be contacted at (408) 568-3734; email: vsolberg123@aol.com.