Thursday, November 06, 2008 | Charles S. Leech, Jr., Director of Engineering, Innovative Drying Co. LLC
Editor's Note: This is the second of a six-part series of columns that will discuss the impact of recent changes to industry specifications concerning moisture sensitive packages and process sensitive components.
SMT Component Assembly: Everything Has Changed
Part Two: You Need to Understand the Specifications
Although mentioned in the previous column, I reiterate that, in this era of economic uncertainty, stiff competition and razor-thin profits, EMS firms have to re-examine their manufacturing processes and eliminate waste at all levels to survive. Since moisture sensitivity level (MSL) issues impact almost all phases of the manufacturing operation it's even more pervasive than electro static discharge (ESD). This can be a terrific opportunity to re-evaluate your entire MSD program and take advantage of opportunities and options to save both money and precious manufacturing time. We will examine these issues in the third column of this series.
A new specification has been added--ECA/IPC/JEDEC Std-075 addresses MSD and process sensitivity level (PSL) of components other than ICs are being evaluated for their sensitivity to the increased reflow temperatures. IPC/JEDEC has recently published revisions to MSD specifications J-Std-020D.1 and J-Std-033B.1.
Read the Specifications
In the previous column, I asked you read these three J-Std specifications.¹ They aren't easy reads. When you are finished, you may feel like you're ready to take the State Bar exam. The relationship between the three documents isn't clear. By splitting the PSL and MSL issues for non-IC components and the MSD concerns for surface mount IC packages you can get a clear idea of each topic. The explanations presented below are not all-inclusive. You will still have to do your homework.
Understanding the Non-IC Component Specifications
Figure 1 shows the relationships between the three specifications that apply to non-IC components.²
Figure 1: Non-IC component, PSL/MSL, specifications flowchart.
1. Passive, Discrete and Non-Integrated Circuit Components: These are non-IC SMD components that may be susceptible to damage from either expanding internal moisture or by excessive solder reflow temperature.
2. J-Std-075: References J-Std-020 for MSL (moisture/reflow sensitivity) determination and determines the (PSL process sensitivity) of non-integrated circuit components. It serves as a guide for component suppliers and assemblers.
--Separate PSL ratings exist for a component depending on whether components are wave or reflow soldered. These are shown in Tables 5.1 and 5.2 of this specification.
--All Moisture Barrier Bags (MMB) that contain these components must have a label that shows the PSL values.
3. J-Std-020D.1: Details the procedures to be used by device manufacturers to classify MSL of non-IC component to damage by expanding ingressed moisture.
4. The "floor-life" for typical factory atmospheric conditions of non-IC components is listed in J-Std-020D.1 Table 5.1 which is the same for solid state (IC) devices.
5. Currently, the initial dry-baking process in J-Std-020D-1 is specified by J-Std-075 to remove ingressed moisture from components. "Bake the sample for 24 hours minimum at 125 +5/-0 °C." This step is intended to remove moisture from the package so that it will be ‘‘dry.'' The dry-bake table (4.1) in J-Std-033B.1 is not used for non-IC components, because components are constructed differently.³ The 125˚C drying temperature places an expensive and time consuming burden on the board assembly industry because components that have exceeded their "floor-life" must be removed from their tape and reel or plastic shipping tube packaging, placed into high temperature JEDEC trays, baked and then returned to new tapes.