Q1: Are we saying then that IMC can occur at the solder / Cu pad join on the PCB when finished assembled? A1: Yes, but in a broader context I am saying that when someone solders correctly, they form an IMC.
Q2: Will the tin diffuse with the Cu over time anyway and in our case start at the join of the Sn/Pb coating and Cu sub-straight on the lead of the device? A2: Exactly, when your supplier fuses, dips, or what ever process to apply that solder to your component leads, they will form an IMC that will continue growing, regardless if you assemble the component or not. When the bare board is HASL, the copper pads on your bare board form an IMC in a separate process. In each of these cases, the IMC takes on a substantial growth spurt when the component is soldered to the board.
Q3: What, in your opinion, is the recognized process for making allowances for the IMC at the PCB Cu pad? A3: With SMT components, recognized process selects pads to have adequate strength, limits the time above solder liquidous temperature, and uses a fairly steep cool-down temperature ramp. With PTH components, there is no allowance, because the action of the wave washes the old IMC away from the solder connection [it becomes an element of your dross] and a new IMC is formed at the copper solder boundary.
Q4: Gold plating, etc etc?? A4: Beyond the issues of Q3, ENIG need to be plated properly. Issues are: * Nickel corrosion. * Thick gold. * Thin gold. * Black pad.
Search the fine SMTnet Archives for discussion on these topics.
Q4: Conformal coating etc etc?? A4: Unaware of conformal coating issues relating to IMC.
Q5: Are we also saying that it would be best to press on and get the other 4/5 of the stock (approx 150,000 devices, approx �0.5 million) packaged? A5: No, I would stay the course. I would NOT commit additional die to packaging, but obviously you need to be: * Reasonably sure that your supplier could package the nonpackaged devices to prevent deterioration. * Confident with your market prospects.
Q6: 20 - 30 C 70% R.H is pretty normal - is it the inert atmosphere that will help most? A6: No. The packaging according to J020 is your first line of defense. Maybe a thicker, rather than a thinner solder coating on the leads is either more important or at worst slightly less important. Finally, I�m suggesting that low temperature storage to limit IMC growth probably has limited benefits and that you probably would be better advised to the spend the money on low temperature store on other remedies, like nitrogen storage.
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