Hello to all.
for certain time, we have been struggling with some issues on a PCB where have tombstoning on 0402 components (due to pad design) and dewetting in qfp256 0.5 mil pitch (discussed on another threads on this forum) component, we have proven that wetting is improved on this component applying nitrogen on our process (I know, is only a patch, but so far is the unique working solution...), BUT on the other hand we have tombstoning issues on our 0402 components while running on nitrogen, this is my scenario:
Solder paste: Indium NC-SMQ92J 0402 components: Stackpole electronics (several P/N) Stencil: Stainless steel laser-electropolish (Interlatin) Pad/Stencil (see attached) QFP: Renesas 256 QFP (90Sn10Pb) Reflow oven: Heller 1809 MK III (9 zones, 3 cooling) Nitrogen parameters: Exit flow: 650 SCFH air Entrance flow: 300 SCFH Cooling flow: 300 SCFH Fan Seal: 300 SCFH
We have been using both RSS & RTS profile, RSS with a soak between 170 & 180 to minimize temp imbalance to avoid tombstoning (recommended by Indium) and a conventional RTS profile (Attached screenshoots), the deal is if I cut down N2 tombstoning is almost eliminated (less than 1%) but we increase wetting issues on QFP and viceversa. I also include the pad design and stencil design to avoid this issue, a new design is coming on but while is implemented we need to live with the issue.
My biggest issue is that I have no idea on N2 settings, heller only toldme "use settings what works for you, i could recommend something but you need to try" in fact there's not a "one size fits all" solution but they didn't mention initian settings, have any of you experienced or do you know about a tutorial where explain how N2 works? I will like to try a optimum point where tombstoning and dewetting is minimum and I will like to consider the following parameters:
1.- Stencil design actual vs "other" 2.- Profile (RSS VS RTS) 3.- N2 settings 4.- Printer stroke speed 5.- Printer system pressure
ideas?
thanks!
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