Is Baking Required After Water Cleaning?

Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, who by day go as ITM Consulting. Jim, what's the question du jour?

Jim
Comes from C.W. "Is baking required after aqueous cleaning? Do circuit board assemblies need to be baked after routine aqueous cleaning? If so, how long and at what temperature?"

Phil
Well, the answer to the first question - NO! Moving right along here. That answers the second question, too, doesn't it? That's pretty good.

Jim
The absorption of moisture by the resins in either components or circuit boards takes place very slowly. So the few minutes blasted with water and a cleaner the exposure is only a couple of minutes.

Think about it this way: regardless of humidity - even in high humidity environment - for packaging and kitting and handling, both components and boards are considered to be okay for an hour out of dry cabin or a moisture barrier bag before you start a clock on them.

The couple minutes in the cleaner does not absorb enough moisture to be significant. It's a long duration over a long period of time.

Phil
This is a question we get asked quite frequently, especially when we're working with clients with regard to MSD. As there's more and more MSD cognizance, it's a question that comes up. And now, thanks to Jim, you know the reason why it's not a concern.

Jim
That was an easy question. Every now and then, we get a break, don't we?

Phil
Thanks for listening to the Board Talk.

Jim
And whatever you do, don't solder like my brother.

Phil
And sure don't solder like my brother.

The thermal profile of the wash process is vital concerning the propensity of circuit boards absorbing water. It is critical that the temperature of the circuit board increase as it travels through the environment of a wash system. This creates molecular expansion, making the board hydrophobic, or less likely to absorb moisture. If a boards temp decreases through the wash it behaves hydrophilic as it contracts, and will absorb the contents of its environment, in this case water. It is critical to profile your wash systems such that board temperatures steadily rise, if only a few degrees, throughout the wash and drying process.

Whoa! It is very important to bake the CCA (note, a CCA is different from a PWB) after wash for the following reasons:

1. The moisture cannot be blown out from inside of connector pins, from under flat ICs, in magnetics windings, etc, etc, etc. You can stand their and blow air on that board all day, but I guarantee there will be some liquid left behind somewhere.


2. If you then place the CCA in a sealed bag with moisture still on it for more than a few hours, you have the perfect recipe for disaster.


3. If you need to perform coating, epoxy application, encapsulation, etc, you will have major issues when attempting to cure.


4. If you need to perform electrical test of any kind, the moisture will skew the test results, especially in high-voltage or high-frequency CCAs.

It is a good idea to quickly blow off as much moisture as possible, and then bake at 105 deg. C. for at least 20 minutes. This will nearly always result in a dry CCA.

If you are dealing with PCBAs that have high voltage circuits on them (such as LV Drives) and you want to put them thru FCT testing soon after washing, then you definitely want to dry them, and dry them well! Especially if they have components that can accumulate significant water (such as bobbin transformers).

In the past I got this exact question from the Test Enginner in an LV Drive Repair department, because they would have to heavily wash very dirty PCBAs that were in customer repair request units. So, I set up an experiment using our very accurate electronic scale (+/-0.05gram) and some PCBAs that were washed. We weighed them and then put them in the Walk-in Thermal Chamber at 70C, and took them out for re-weighing every 2 hours. We did this for 10 hours. The results showed that after 6 hours the weight curve was flat, so we set the drying time at 8 hours to be on the safe side.

But on average the PCBAs had lost 1.5 CC's of water in that 6 hours! So, you have to ask yourself; do I want to put 480VAC on a PCBA soaked with 1.5 CC's of water?? Another reason to do post-washing drying is if you are going to encapsulate (pot) the PCBA? If your are then drying is a must! And another good trick is to ramp down the drying temp from 70C to about 40C (for safe handling) at the end of the chamber cycle, and then take the PCBAs out one at a time, so that when you apply the Potting the PCBA is still warmer than the surroundings and condensation will not form on the surfaces.

Of course the Potting Room must be temp and humidity controlled and the material should be applied using a good vacuum degassed system. And another good reason to dry the PCBAs is if you are still using glue deposits under SMD parts (using waved solder instead of double-sided reflow) and then conformal coating the PCBA. The wash water gets stuck in the cavities of the glue and then when you conformal coat over these parts the water is pretty well captured. This can/does result in dendritic failures, especially if you left a lot of "copper fill" between the pads and you have some "component shift".

Recommendation: Remove all copper-fill between pads of flush mount SMD parts and use double-sided reflow...eliminate the glue!

You can refer to JEDEC standards for baking procedures. Usually the baking is not required. I have to agree with others though that it depends on multiple factors:

• What is the population on the board (can water be entrapped in certain areas)?


• How much time is the assembly going to be in the ambient before the next process step and in what type of environment?


• What is the next process step? (E.g. conformal coating materials "like" clean and dry assemblies!!!)


• What is the material that the PCB is made out of (hygroscopic properties)


• Is the cleaning performed in an in-line or batch washer?


• Are there any chemicals used?


• Are there any special requirements from the customer?

The ideal situation is a moisture free assembly before any dramatic temperature change like a reflow process. Cold weather can also be an issue - few drops of water in the ESD bags that the assembly was shipped is not unusual for the boards that were not dried properly before shipping.

Jim's comment "The couple minutes in the cleaner does not absorb enough moisture to be significant." What if CCAs spend 40 minutes in the cleaner?

I agree completely with your short exposure times answer to water wash, my concern would be the difficulty of natural drying or machine hot air on exit drying, when the wash is with suponifiers, they typically penetrate deeply and are not easily driven out post wash process especially on low profile components.

The answer to that question should say depends on what parts the customer is using in W.S. process, for example: if using Xilinx flip chip you do have to bake assembly after wash.

Like most things in life.. it depends. Post bake is most certainly required if subsequent solder paste reflow, rework station, and/or wave solder processes follow the water wash operation. PCB delamination and component "pop-corning" can easily occur due to rapid vaporization of the water from extreme thermal excursion. Refer to JEDEC standards for baking procedures.

Post bake is generally not required after water wash as the last operation of the manufacturing process. However, if you have components with cavities, pockets (such as some connectors) care should be taken to make sure the assembly is completely dry prior to packaging.

Majority of the water ingress related problems are not with the PC Board Laminate, but the integrated circuit packages. The ingress normally starts at the interface of the packaging with the leadframe, and penetrates to the die in case of differential stress between the lead frame and the packaging material. The failure due to moisture ingress in higher power components will be evidenced by popcorning of the package.