WHITE PAPER REPORT :

Transcription

1 ENGELMAIER A SSOCIATES, L.C. Electronic Packaging, Interconnection 7 Jasmine Run, Ormond Beach, FL and Reliability Consulting Phone (386) Engelmaier@aol.com Web Site: WHITE PAPER REPORT : Recommendations for PCB FAB Notes and Specifications in Printed Circuit Board Drawings for SnPb and Lead-Free Soldering Assemblies, the Qualification of PCB Shops and Activities to Assure Continued Quality In this report recommendations are made regarding appropriate specifications and FAB Notes on drawings for printed circuit boards (PCBs), general procedures to qualify PCB shops and to assure they would be producing PCBs of good quality, and testing procedures to verify quality and reliability. The current report contains examples one for SnPb solder assemblies and one for Pb-free solder assemblies of FAB Notes serving as general specifications on PCB drawings, a basic questionnaire for new PCB shops to be qualified, as well as recommendations for ongoing activities to assure that qualified PCB shops maintain the quality of the PCBs produced by them. Generally speaking, too much is left to the discretion of the PCB manufacturer. This can result in good quality PCBs for high-quality circuit board manufacturers and significantly less quality from some of the cheaper PCB shops. However, even PCBs with good quality can exhibit inadequate reliability because the specifications were simply inadequate for the assembly processes or application operating environments. This is particularly the case when the PCBs are subjected to multiple soldering operations and/or to the higher solder process temperatures necessary for lead-free solder assembly. This report/multi-client study is the intellectual property of Engelmaier Associates, L.C. Any reproduction and/or dissemination of its content outside the organization that purchased this report is prohibited.

2 ENGELMAIER ASSOCIATES, L.C. 10/ 03/ 06 Page 2 of 26 R e f e r e n c e s 1. EXAMPLES OF SETS OF FAB NOTES SERVING AS PCB DRAWING SPECIFICATIONS FOR PCBs 1.1 FAB NOTES FOR PCBs FOR TIN/LEAD SOLDER ASSEMBLIES 1.2 FAB NOTES FOR PCBs FOR LEAD-FREE SOLDER ASSEMBLIES 2. EXPLANATIONS/DISCUSSION OF FAB NOTES /SPECIFICATIONS 2.1 NOTES PB-1/LF-2 Acceptance 2.2 NOTES PB-2/LF-3 PCB Type 2.3 NOTES PB-3/LF-4 PCB Class 2.4 NOTES PB-4/LF NOTES A & B PCB Laminate/Prepreg Material The PCB Base Material and the Prepreg Material are to be specified according to the PCB requirements. Industry document IPC-4101 [Refs. 1/2] is the basic document for this purpose. It should be noted, that specifying IPC-4101 slash sheets is not sufficient to specify specific properties. The ranges of properties contained within these slash sheets are too broad for this purpose; therefore it is recommended, that specific materials and their equivalents be specified for critical applications. Whether to use Dicy-cured (Dicyandiamide) FR-4 materials is being debated in the industry. Some are adamant about not using Dicy-cured materials in highdensity high-reliability applications, while others have good experience and like the economy achievable. The glass transition temperature should be determined using the Thermo- Mechanical Analysis (TMA) method as per IPC-TM-650, C [Ref. 3]. The TMA method is preferred over the other two methods sometimes used to determine the glass transition temperature, DSC [Ref. 4] and DMA [Ref. 5], because the thermal expansion of the PCB is a critical parameter, which is given by TMA as a function of temperature. 1. IPC-4101 Laminate/Prepreg Materials Standard for Printed Boards, The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, December IPC-4101A Specifications for Base Materials for Rigid and Multilayer Printed Boards, The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, June IPC-Test Methods Manual, IPC-TM-650, C, Glass Transition Temperature and Cure Factor by DSC, The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, December IPC-Test Methods Manual, IPC-TM-650, C, Glass Transition Temperature and Z- Axis Thermal Expansion by TMA, The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, December IPC-Test Methods Manual, IPC-TM-650, , Glass Transition Temperature and Thermal Expansion of Materials Used In High Density Interconnection (HDI) and Microvias-DMA Method, The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, November 1998.

3 ENGELMAIER ASSOCIATES, L.C. 10/ 03/ 06 Page 3 of 26 The MIL-Spec designations, where available, are included for reference and are defined as follows: GNF/GFG stand for E-glass, woven [G], epoxy, flame resistant [F], natural color [N]/green color [G]; PGFN/PGFG for prepreg [P], otherwise the same as for the laminates. For PCBs subject to soldering processes using the more elevated soldering temperatures required for lead-free solders, specifying a glass transition temperature, T g, of 140 C will not be adequate for PCBs thicker than about 50 mils. Furthermore, for thicker PCBs, it is recommended that a minimum decomposition temperature, T d, determined as per IPC-TM-650, [Ref. 6] as well as a maximum thermal expansion coefficient in the PCB thickness direction, CTE(z), determined as per IPC-TM-650, [Ref. 7] be specified. CTE(z) values should be given separately for temperatures below T g, 1, and above T g, 2 ; however, frequently the thermal expansion, TE in %, is lumped together from 50 to 260 C or even 50 to 288 C. Typically, the decomposition temperature is given as T d (5%) to a 5% weight loss; the decomposition temperature, T d (2%), to a 2% weight loss, has been found a very good indicator, but is not as yet widely available. Frequently, the time to delamination, either T-288 or T 260 [Ref. 8] are specified either in addition to T d or instead of it. The T-288 delamination time provides a more appropriate level of performance given the process temperature required for LF-soldering. The delamination time is sometimes combined with the requirement that that temperature needs to be survived for 4 to 5 excursions. Unfortunately, the data sheets from the various laminate/prepreg suppliers are neither consistent not complete. What is worse, on the occasions where property measurements were performed by commercial laboratories because the values given in the data sheets did not appear to be credible, values of the CTE(x) and CTE(y) were found to be nearly double those given in the data sheets. The easiest way to specify the three properties critical for the survival of the PCB and the PTH/via interconnect structure T g, T d, thermal expansion (TE), is by specifying a minimum Soldering Temperature Impact Index, STII, which is defined as STII = T g /2 + T d /2 (TE%(50 to 260 C) x 10). For PCBs with thicknesses of 0.06 inches [1.5 mm] or more, an STII-value of 215 or larger is recommended. However, the STII-concept is not widely used as yet. Table I lists some of the more prominent resin systems, their pertinent properties, and the resulting STII-values. Wherever the needed property was not given in the data sheet, the value used is designated with ~. In Table I, Laminates with Limited Lead-Free Compatibility means PCBs of thickness limited to about less than 60 mils [1.5 mm]. The vendors listed in Table I 6. IPC-Test Methods Manual, IPC-TM-650, , Decomposition Temperature T d ) of Laminate Material Using TGA, The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, April IPC-Test Methods Manual, IPC-TM-650, , Coefficient of Linear Thermal Expansion of Electrical Insulating Boards, The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, March IPC-Test Methods Manual, IPC-TM-650, , Time to Delamination (TMA Method), The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, December 1994.

4 ENGELMAIER ASSOCIATES, L.C. 10/ 03/ 06 Page 4 of 26 provide data sheets for the laminates with sufficient technical information; others certainly do exist, but the available information was inadequate to calculate STII-values. Table I. Various Laminates/Prepregs with their Properties and STII-Values. Vendor Material Designation Glass Transition Temperature by TMA, T g ( T M A ) Decomposition Temperature, T d ( 5 % ) High-End Standard Laminates Thermal Expansion (50 to 260 C), TE(50 260) S T I I TUC/Taiwan TU C 310 C 4.1% 181 ISOLA IS C 330 C 3.0% 202 NELCO N C 330 C 3.7% 203 Laminates with Limited Lead-Free Compatibility TUC/Taiwan TU C 340 C 3.4% 209 ISOLA FR250 HR ~140 C 350 C 3.4% 211 Lead-Free Compatible Laminates TUC/Taiwan TU C 330 C 3.5% 216 TUC/Taiwan TU-642 ~135 C 360 C 2.7% 220 TUC/Taiwan TU C 370 C 3.1% 224 ISOLA IS410 ~170 C 350 C 3.5% 225 NELCO N C 345 C 3.2% 225 ISOLA FR408 ~170 C 360 C 3.5% 230 NELCO N C 350 C 3.0% 232 TUC/Taiwan TU C 350 C 2.7% 233 ISOLA 370 HR ~170 C 350 C 2.7% 233 NELCO N C 370 C 3.6% 239 Advanced Lead-Free Compatible Laminates ISOLA IS415 ~180 C 370 C 2.9% 246 NELCO N C 365 C 3.5% 247 TUC/Taiwan TU C 340 C 2.5% 250 ISOLA IS620 ~205 C 353 C 2.8% 251 ISOLA IS500 ~170 C 400 C 2.8% 257 TUC/Taiwan TU-842 ~170 C 390 C 2.1% 259 POLYCLAD LD-621 ~190 C 400 C 3.0% 260 Instead of specifying specific material properties, some in the industry prefer to specify the capabilities of the finished product. The argument advanced is that not only the properties of the raw materials used, but the possible impact of processing are important.

5 ENGELMAIER ASSOCIATES, L.C. 10/ 03/ 06 Page 5 of NOTE C Lay-Up NOTES D & E Copper Foil 2.5 NOTES PB-5.A/LF-6.A Minimum Copper Thickness 2.6 NOTES PB-5.B/LF-6.B Thieving 2.7 NOTES PB-5.C/LF-6.C Surface Treatment 2.8 NOTES PB-5.D/LF-6.D Solder Mask 2.9 NOTES PB-6/LF-7 Non-Functional Lands 2.10 NOTES PB-7/LF-8 Etchback 2.11 NOTES PB-8/LF-9 Conductor Width 2.12 NOTES PB-12/LF-13/HOLE CHART Drilling Guidelines 2.13 NOTES PB-13/LF-14 Number of Drill Hits 2.14 NOTES PB-14/LF-15 Coupons 2.15 NOTES PB-15/LF-16 Copper Plating Bath Chemistry 2.16 NOTES PB-16/LF-17 Quality of Copper Plating 2.17 NOTE LF-22 Marking and Labeling 2.17 NOTES PB-24/LF-25 Packaging and Shipping 3. QUALIFICATION OF PCB SHOPS 3.1 PCB SHOP QUESTIONNAIRE Laminating Process Drilling/Desmear Process Wet Processes/Plating Processes Pack aging/ Shipping 3.2 QUALIFICATION VISIT 4. ACTIVITIES TO ASSURE CONTINUED QUALITY 4.1 ONGOING QUALITY ASSURANCE/REQUALIFICATION 4.2 INCOMING INSPECTION 5. CAUSES OF PRINTED CIRCUIT BOARD FAILURES