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IT'S DIFFICULT, IF NOT NEAR IMPOSSIBLE, TO PROCURE ELECTRONIC ASSEMBLY ...

By Edith Duncan,2014-07-08 11:19
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THE FABRICATOR CAN SUPPLY ROHS AND LEAD-FREE WITHOUT MAKING A PCB THAT WILL ... THE NOTE MUST ALSO STATE THAT THE PCB SUPPORT A LEAD-FREE ASSEMBLY PROCESS AND ...

Whose RoHS Definition Do You Use? And, Why Are There Different Definitions?

By:

    Stanley L. Bentley, P.E.

    Senior Technical Advisor, Divsys International

David Fissell MBB

    Principal, Global Lean Consulting

    Procuring printed circuit boards (PCBs) was relatively simple when all you had to do was throw a few acronyms into drawing notes such as, “FR-4 and SMOBC with HASL”. As time has past, simple

    drawing notes have become not only inadequate; they are ambiguous and lack sufficient clarity.

A common practice is to state in the printed circuit board engineering design drawing notes “PCB shall be

    RoHS compliant”. One belief is that invoking RoHS will guarantee that the product material is lead-free

    and all lead-free material is lead-free assembly capable. RoHS compliant product means, in the purest sense, the materials used in PCB fabrication are lead-free and virtually bromide free (only).

    There are three stakeholders in the electronic product chain, the PCB fabricator, the Assembler, and the OEM. It is critical to determine to whom a general RoHS drawing note is addressed and how they perceive their compliance with this note, to assure the expected product is received.

    Why does the PCB fabricator have the most latitude with a general RoHS compliance drawing note?

    From a PCB fabricator’s perspective, they could take the position that there is no difference between an engineering note “PCB shall be RoHS compliant” and a note that calls out “PCB shall be lead-free”. The

    fabricator must be given more detailed instruction to ensure the requirements of the OEM (who is not normally his customer) are met. The fabricator can supply RoHS and lead-free without making a PCB that will support a lead-free assembly process. In other words, RoHS compliant alone is not an adequate requirement. The note must also state that the PCB support a lead-free assembly process and provide some definition of the material to be used. The less restrictive note requires PCB fabricators to choose a RoHS compliant material and the latter to choose a lead-free material that will survive a lead free assembly process.

What does lead free assembly mean?

    It’s difficult, if not near impossible, to procure electronic assembly parts with surface finishes other than lead-free. RoHS compliance has moved electronic hardware into the age of lead-free. Even industries that are technically “exempt”, must deal with the reality that the components will be lead-free, and that

    assembling with a tin/lead process can be problematic in attaching to these components. Further, if the PCB has a lead-free finish and the assembler is using a tin/lead paste to attach to lead-free components, the process issues abound.

    Why does the assembler care if the PCB is capable of lead free processing as long as it is RoHS compliant?

    He cares because it has a direct impact on his yield and his rework costs. Many epoxy-glass FR-4 laminate materials are unable to survive, without delamination, the higher lead free soldering temperature (240 ºC 260 ºC). With higher lead free soldering temperature, the resin system’s material thermal

    stability Thermal Decomposition Temperature (Td), Z-axis CTE (Alpha 1 and Alpha 2) below and above the Glass Transition Temperature (Tg), and Time-to-Delamination (T260, T288, and T300) - become critical factors in choice of FR-4 laminate material.

    The assembler is most concerned about de-lamination and coefficient of thermal expansion. De-lamination creates a total loss of the value of the assembly, while CTE issues can create unsoldered connections, billboarding, cracked components, and other defects that create scrap and rework.

    To ensure good yields, Electronic assemblers, at minimum, desire the FR-4 laminate materials have low moisture absorption (? .5%), a Td ? 325?C, Z-axis CTE ? 3.5% from ambient to 260?C. Further, the

    PCB must be able to survive 5 assembly simulation reflow excursions with a peak temperature of 288 ?C and still exhibit acceptable reliability. The above conditions reduce electronic assembler’s risk associated

    with a delamination during assembly process.

    PCB materials supporting lead free assembly process (LFP) are more expensive than traditional PCB materials. LFP material properties are a higher grade. LFP materials are harder and more brittle, thus reducing drill hits and increasing drill costs. LFP materials lamination recipes are longer, reducing the fabricator’s capacity. When all of these factors are taken into account, the fabricator experiences a cost

    increase for LFP of approximately 20% over traditional FR-4 substrates. This cost variance creates a disincentive for the fabricator to volunteer to produce LFP printed circuit boards without a clear directive to do so.

Why does the OEM care?

    The OEM has a regulatory requirement to meet RoHS with their end product. However, de-lamination that is not detected by the assembler will potentially cause an early field failure, promote metal migration or other conductive path failures. At a minimum, the electrical properties of the dielectric medium are degraded. All of these can create unwanted field returns.

    How do we communicate a material requirement to the fabricator that satisfies the needs of both the assembler and the OEM?

    The easiest way to communicate the material information needed by both the PCB fabricator and electronic assembler is to state a specific IPC-4101 slash sheet, such as 4101/124, required. The

    drawing note becomes “PCB shall be RoHS compliant, use 4101/124 laminate material.” Or, a less

    restrictive note could state the PCB shall be lead free, capable of meeting a lead free assembly process. The risk with the less restrictive note is there is not a universal definition of the lead free assembly process.

    An obvious universal “fix” to the material issue is to specify the highest grade. However, few designs can absorb the significant cost penalty for this option. To minimize the cost, the OEM should choose the laminate material based on meeting their unique performance requirements while minimizing the potential for de-lamination or yield issues during assembly. Because this process involves discussions with (both) the fabricator and the assembler (time consuming), the solution commonly chosen is often to use a general drawing note, such as “PCB shall be RoHS compliant”. Thus the miscommunication of semantics begins…

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