|Ask the Experts
November 8, 2022 - Updated
November 2, 2022 - Originally Posted
Baking Old PCBs Prior To Reflow
Our standard procedure for new bare PCBs is to bake at 125C if the date code is greater than six months old. These PCBs are sealed in plastic from the supplier. The packages have desiccants and indicator cards with moisture levels below 10%. I understand opened packs, but all of these have been sealed. To avoid delamination during reflow, is it required to bake these PCBs before use? We have reviewed J-STD-033 and found no reference to this issue.
|Expert Panel Responses
I will go ahead and bake just for good measure. Moisture hides in weird places like inside plated through-holes.
Senior Manufacturing Engineer
If sealed, desiccated, and monitored with indicator cards, there’s no need for baking older PCBs as mentioned in the question. Not only is it unnecessary, but if not baked in an inert atmosphere, certain surface finishes will oxidize making soldering more difficult and/or jeopardizing solder joint quality/integrity.
The life span of a PCB is not only about the exposure to humidity, and not everything can be solved by baking. The finish of the best PCB's allow them shelf life assuming they were sealed and stored in a controlled environment. I suggest to follow the IPC to the letter, as the quality of the finish can affect the quality of the product. It is also a good idea to have an additional visual inspection and look for evidence of corrosion or increased roughness of the pads.
I would NOT make it a hard fast “rule” to always bake boards. Baking can exacerbate oxidation (IPC J-STD-033D 188.8.131.52) on solderable finishes. Sealed dry packaging is good for 1 year (IPC-1601, 5.2). If the 10% dot on the HIC has had significant color change, the seal has been compromised (IPC-1601, 5.3) and then baking may be useful/required.
Storage of MSL components (including bare boards/PCBs) in a dry cabinet <=5%RH, is considered equivalent to storage in a dry pack with unlimited shelf life (IPC J-STD-033D, 184.108.40.206).
Senior Project Engineer
Electronic Controls Design Inc
You are baking the boards to ensure that there is no moisture, if they are sealed and all the moisture indicators say that there is no moisture then you should be OK. But just to be on the safe side you can always run a bare board through your reflow to see if it is OK.
Pre-baking can also induce a risk of warpage on the boards so you might want to consider this as well when factoring in what you want to do.
Global Product Champion
You did not specify what the base material is for the PWB- FR4 vs polyimide for example. If your supplier has baked the boards to remove all moisture prior to shipment and you have left them in their original moisture proof packaging then it might be redundant to bake again. However if you cannot guarantee that the supplier has done this then it is a major risk, especially for polyimide type materials and you need to continue baking.
S T and S Testing and Analysis
The temperature and duration to which a bare PCB has to be subjected to baking depends on the thickness of the PCB and the conditions of storage. Here also one has to observe time-temperature relationship. Generally, for a 2mm PCB it is 110oC for 4 hours or 93oC for 8 hours. The packages need to be stored in vacuum sealed/ nitrogen sealed moisture barrier bags.
A level of not greater than 10% RH with the plastic sealing is also bound to absorb moisture, unless it is vacuum/nitrogen sealed. Moreover moisture ingress can happen once it is opened for inspection, phases of movement for reflow soldering, post-reflow inspection, manual soldering of relays, leaded devices etc.
There are various reasons for delamination other than humidity build up such as - Construction of PCB w.r.t resin content, types of fiber glass cloth used, adhesion and long term thermo chemical compatibility between the resin system and copper, impact on CTE both in z-axis and y-axis, a very high resin content pre-preg layer adjacent to very-low resin content layer, resin-rich and glass-rich areas within a pre-preg area, excess heat etc.
To minimize the entrapped moisture it is always advocated to carry out baking as given above. Moreover, for any soldering operation on the card whether it is manual soldering or rework the cards shall be properly conditioning by thermal baking.
Vikram Sarabhai Space Centre
You didn’t mention what type of base material or surface finish the PCB’s have. Especially the surface finish of the solder pads are interesting since this may affect the shelf life. From my point of view I would not use a standard procedure to bake everything that has a date code older than 6 months.
Most surface finishes are specified to handle at least 12 months of shelf life in unopened packaging (with desiccants and humidity indicator card). Two relatively common surface finishes, OSP and Immersion Ag, are specified with the 6 month limit in shelf life (however some PCB manufacturer states that Immersion Ag can handle 12 months as well).
So - depending on the surface finish of your PCB and given that the PCB packaging are unopened, not damaged in any way, and that the humidity indicator doesn’t show any signs of humidity in the packaging. If it is not for example OSP or Immersion Ag as surface finish, I would not bake these PCB’s after 6 months. To be safe I would ask the PCB supplier for the shelf life they specify for the type of surface finish you use on your PCB’s.
Senior Quality Engineer - PCB Assembly
If the boards are sealed with desiccant and the humidity indicator card is below 10% you shouldn’t have to bake the boards. Repeated baking can oxidize the surface finish and increase the intermetallic growth before the board is soldered. If the board goes through multiple reflows and a rework or two the intermetallic growth may be a concern.
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
Refer to IPC-1601 for answers to all questions regarding baking of circuit boards, including methods for determining optimum bake times and temperatures. You do NOT need to bake at 125C, the experts are correct in that baking at that high of a temperature will degrade solderability somewhat. 105 deg. C is a good general baking temperature that will work for 99.9% of all PWBs. J-STD-033 is intended for baking of components only.
If there is no concern regarding moisture presence in PCBs, I consider the process unnecessary and even harmful as you can create issues on the exposed metal finishes.
Engineering and Operations Management
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