Issue link: https://iconnect007.uberflip.com/i/1284035
SEPTEMBER 2020 I SMT007 MAGAZINE 25 small gap at the overmold/lead frame interface (Figures 6 and 7). Even if the residues don't make it all the way to the die, they can be pres- ent on the edge of the package body between the leads and propagate electrochemical migra- tion (Figure 8). If you are using a fully no-clean assembly process, you can't rely on an end of the assem- bly wash process to remove any of those res- idues. This is when you can use IC analysis to determine the effectiveness of the compo- nent wash process. Standard bag extractions will detect any elevated levels of ionics on the outer surfaces. For internal surfaces, Parr Bomb analysis is a pressurized extraction for harvesting possible plating residues that have been absorbed into the component overmold material, down to the lead frame. This can be done without any physical damage to the com- ponent. Much like the bare boards, compo- nents bring their own inherent reliability risk before the first part is soldered. Now that you know how those raw parts can impact your reliability, let's put all those pieces together. Per IPC J-STD-001 Section 8, you need to qualify an assembly process using SIR per TM-650 2.6.3.7 on test boards to show how well the CM is processing the proposed set of materials and what impact elevated heat and humidity have on electrical resistance. This test is the bare minimum that needs to be com- pleted to verify the assembly process. Contract manufacturers need to do this testing to gen- erate objective evidence that can be applied to process monitoring analysis. As most people know by now, the historical acceptance criteria of 1.56 µg NaCl equivalence per square centi- meter has been given the old heave-ho, and rightfully so. If you want some details on how and why that criterion was removed, I recom- mend reading IPC-WP-019: An Overview on Global Change in Ionic Cleanliness Require- ment. (Spoiler alert: It should never have been used as it has been.) Here is an example of how a CM can gen- erate objective evidence and use it for pro- cess monitoring. If a company needs to qual- ify a product with a new customer, and the Figure 6: Gap at overmold lead frame interface. Figure 8: Dendrite growth on package body. Figure 7: Dendrite on package die.