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Floor life pcb assembly

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#54459

Floor life pcb assembly | 23 April, 2008

Hi all,

Can anyone advise me what is the recommended floor life for pcb assembly prior to wave soldering before drying/degassing ?

Thks

Joseph

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#54461

Floor life pcb assembly | 23 April, 2008

Floor life of a PCB is rated in years. Are you asking about the solder paste or something else?

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#54471

Floor life pcb assembly | 23 April, 2008

The need to prebake boards to remove moisture from the board [to prevent voiding] prior to wave soldering is driven by the thickness of through hole plating. * 1 thou [25um] or greater requires no prebaking * LT 1 thou [25um] requires prebaking

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#54475

Floor life pcb assembly | 23 April, 2008

It depends on the laminate but a PWB is like any other MSL component. Some fab shops are only waranting standard FR4 laminates for 45-60 days when processed at lead free temperatures.

The best thing I can say is to try some weight gain/loss experiments. This will also help you with your bake out profiles.

Take a fresh board and weigh it right out of the sealed and dried packaging (or bake one out fully, e.g. 24 hrs @110C). Then weigh it again the next day, and so on until the weight stabilizes. You now know the rate of moisture absorbtion. Then do the same experiment in reverse. Bake it at 110C and measure every hour or so until it stops losing weight (i.e. all the moisture is driven back out) Now you know what is really necessary for bake out. Not 'some more is better' committee decision that only serves to oxidize the pads.

Somewhere between 30 and 50% of the max moisture absorbtion should be your limit depending on your preheat abilities and how conservative you want to be.

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#54491

Floor life pcb assembly | 25 April, 2008

Hi all,

I mean after post adhesive oven curing (SMD at bottom side) prior to manual insert of PTH components (top side), what is the recommended storage time prior to wave soldering in order to avoid pin/blow holes and sunken joints issues (pcb assembly being stored at <25 deg C and < 60% RH room temp.)

FYI, our PCB is double-sided with 1.6mm thickness with FR4 material.

Thks Joseph

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