Issue link: https://iconnect007.uberflip.com/i/1361971
APRIL 2021 I PCB007 MAGAZINE 97 situation. e supplier should warn you, but not if the design meets their capabilities. 3.4: "Printed Board Dimensional Require- ments SHALL be defined as per IPC-6012E 3.4 unless otherwise AABUS." You cannot come af- ter delivery and call for tighter tolerances, or you may end up with a higher unit price based on requirements revealed in the EQ process. 3.4.2: "Annular Ring and Breakout exter- nal: e use of teardrop designed pads for class 3 SHALL be AABUS. e same is 3.6.2.9 for inner layer pads." If you fail to add teardrops, you could end up with inner layer trace-to-pad junction breakout. is could be a true reliabil- ity issue causing field error. 3.4.3: "Bow and twist requirements for the ar- ray SHALL be AABUS." is is probably the most overlooked rule in IPC-6012. Most of us believe the bow and twist rule is for the cus- tomer panel but it is not, unless this is part of your requirement specification. is also means panel frame design is critical to secure a flat panel even aer the first soldering pro- cess. And, if you shall be 100% sure, you should specify that bow and twist assessment shall be done aer one thermal process. is is even more important if the construction is unbal- anced, as mentioned in paragraph 2.3.4, and such requirements will lead your supplier to check the construction. 3.6: "Structural Integrity: A/B coupons shall be used unless AABUS." is paragraph also leads us to the evaluation of Table 4-2 sampling plan and Table 4-3 acceptance testing and fre- quency. e supplier will probably not use A/B coupons unless you specify it, or if the sup- plier has a focus on defense and other indus- tries where customers typically have a focus on these coupons. Coupons are widely used in the U.S., less in Europe, and very seldom in Asia. at means you need to specify if you want the IPC A/B coupons to be used. I would argue for using the PCB itself for lot qualification micro- sections. 3.6.2.11: "Plating thickness. Deviations to Ta- bles 3-3 through 3-6 of IPC-6012E SHALL be AABUS." is may very well happen in tight HDI designs! In such cases it is important that your design calculates for thinner copper, and that this is agreed upon with your supplier. 3.6.2.11.3: "Requirements to bumps and dimples in copper filled microvias SHALL be AABUS." It is quite strange that IPC has chosen not to specify the requirements to a flat surface of copper-filled blind vias. e re- quirement is there for resin-filled vias, so why not for copper-filled? If your assembly requires a flat surface, you should specify the require- ment, or you can refer to IPC-6012EM, the Medical Addendum to IPC-6012, where this is specified. 3.7.1 f: "Solder mask encroachment on rect- angular SMT pads with pitch less than 0.65 mm SHALL be AABUS." Be aware of the challenge to avoid encroachment on rectangular SMT pads, especially when pitch is less than 0.65 mm. Many customers are not aware of this lim- it in the standard. 3.7.3: "Solder mask thickness requirements SHALL be AABUS." is is the AABUS most used in PCB specifications and corporate re- quirements. It is damaging for solder paste printing when solder mask gets too thick, and of course we fear exposed copper if it gets too thin. 3.9.2: "Cleanliness testing aer solder mask only when specified." e standard asks the de- signer and buyer to specify if such a test is re- quired. e challenge is, the test method speci- fied in IPC-6012E, IPC-TM-650-2.3.25 was de- signed for process control, and not a proof test of cleanliness on the finished board. I suggest using the specification in IPC-6012DA (revi- sion E comes in 2021). 3.11: "Repair—the allowance and require- ments for repair of bare printed board SHALL