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Moisture Bake-out of Plastic Parts

Peter H. Cote

#15736

Moisture Bake-out of Plastic Parts | 26 May, 1998

We currently bake plastic SMT parts in accordance with the IPC guidelines for moisture sensitive parts. Because we have double-sided boards that are subjected to either an aqueous or semi-aqueous cleaning process, we do a 24 hour bake at 125 deg. C after the first side of the board is reflow solder and cleaned. Does anyone know of any studies to indicate either of the following: 1. Is baking after the first side of the board is solder and cleaned really necessary if the board is processed within the time frame recommended for moisture sensittive parts? 2. Does vacuum baking reduce the cycle time for moisture bakeout? If yes, what vacuum ovens are recommended and what is the bakeout schedule? Thanks,

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Steve Gregory

#15741

Re: Moisture Bake-out of Plastic Parts | 26 May, 1998

Hi Peter, There's been a lot of discussion on this topic lately (baking parts), and that got me reading the IPC-SM-786A, which is pretty good. For your first question, the parts absorb moisture over time. The type of package mold compound will determine exactly how much moisture is absorbed over a certain time in a given humidity. IPC has given some general guidelines in a worse case scenario for certain environments, without taking into account the different mold compound types. I would say that you should use the 60% Relative Humidity and 30 degrees centigrade rule, and then treat them as a level 5 or 6 moisture sensitive part, which allows 48 or 24 hours out of a moisture barrier bag. You can extend that time by storing the partially completed assemblies someplace where relative humidity is less than 20%. According to the 786A manual, when you do rework, you should use the same rules too. I only mention that because that's something that a lot of people forget about. As far as vacuum ovens, I've got to admit I don't know much about...it doesn't talk about vacuum ovens at all in the 786 manual...does vacuum really reduce the time required to bake? C-ya! -Steve Gregory-

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Earl Moon

#15740

Re: Moisture Bake-out of Plastic Parts | 26 May, 1998

| We currently bake plastic SMT parts in accordance with the IPC guidelines for moisture sensitive parts. Because we have double-sided boards that are subjected to either an aqueous or semi-aqueous cleaning process, we do a 24 hour bake at 125 deg. C after the first side of the board is reflow solder and cleaned. | Does anyone know of any studies to indicate either of the following: | 1. Is baking after the first side of the board is solder and cleaned really necessary if the board is processed within the time frame recommended for moisture sensittive parts? | 2. Does vacuum baking reduce the cycle time for moisture bakeout? If yes, what vacuum ovens are recommended and what is the bakeout schedule? | Thanks, I don't know if I'm reading you right, but it seems excessive to bake after first side soldering. It seems more likely to bake after completing assembly as moisture is gradually built up over time. Also, why clean after first side assembly? Can't this be done as a final process provided acceptable product is determined? A big concern, to me, is a 24 hour (or any extended bake) of assemblies at 125 C.. Remember, if you are using standard epoxy boards, the typical Tg is from 120 to 130 C. and you know what happens when these resin systems are subjected to extended thermal excursions above their Tg. Earlier in the forum, it was said the reason for vacuum ovens is to reduce the temperature needed for moisture removal. I don't know about the time required but Steve said all the rest. Earl Moon

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Wayne

#15739

Re: Moisture Bake-out of Plastic Parts | 26 May, 1998

Vacuum Bakeout Ovens For many years vacuum bakeout ovens have been used in the hybrid industry for removal of moisture from ceramic cercuits prior to hermetically sealing packages. In this process the vacuum oven certainly reduces the bakeout cycle and removes more moisture particulate. In most cases the process is attempting to reach 20 militorr or lower vacuum levels. Many people believe that if you can reach a point that there is little or no outgasing you have reached the level possible with a standard roughing pump. The next stage would be to go to a turbomolecular type pumping system which would increase pump down time and mositure removal capabilities. The oven chamber and the vacuum port size determines the amount of outgas particulates that can be removed from the chamber. Heat increases the movement of the molecules causing them to move around in the chamber and at some point they will reach the vacuum port. This is where the turbomolecular pump comes into play as the molecules enter the chamber they are forces out by the turbo pump blades. The problem is the size of the oven requred for PCB's I am not too sure what ovens are available and to what vacuum pressure they can achieve. As the size of the board increases so does the vacuum oven chamber, roughing pump and vacuum port size. It also requires batch processing so if there is an oven chamber large enough then the vacuum process would certainly reduce the backout cycle time, but at a cost that might not fit your budget. There are several companies that manufacture vacuul pumps such as Edwards and Alcatel and they could assist you in pump and chamber sizing. Wayne

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Justin Medernach

#15737

Re: Moisture Bake-out of Plastic Parts | 26 May, 1998

| We currently bake plastic SMT parts in accordance with the IPC guidelines for moisture sensitive parts. Because we have double-sided boards that are subjected to either an aqueous or semi-aqueous cleaning process, we do a 24 hour bake at 125 deg. C after the first side of the board is reflow solder and cleaned. | Does anyone know of any studies to indicate either of the following: | 1. Is baking after the first side of the board is solder and cleaned really necessary if the board is processed within the time frame recommended for moisture sensittive parts? | 2. Does vacuum baking reduce the cycle time for moisture bakeout? If yes, what vacuum ovens are recommended and what is the bakeout schedule? | Thanks, Pete, Not to pry but whatcha buildin'? If that's proprietary, no big deal. The reason I ask is because when I have built stuff for the medical industry, for example, I have to follow rigid process guidelines. Sometimes, even some consumer electronics customers designate processes. My point is this; You don't need to bake after the first component population / reflow. I never did and I never had a problem. I've built about a bazillion different assemblies too so my statement is not lacking the backing. I think Wayne pretty much nailed the vacuum oven part. I used regular convection / baking ovens when trying to clear moisture from parts. You DO want to wash after your first SMT assembly operation. I assume you're running a water soluble process. Not washing would more than likely turn your flux residue into something just short of concrete if you you pass that stuff through a second reflow profile. Cheers, Justin

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Earl Moon

#15738

Re: Moisture Bake-out of Plastic Parts | 26 May, 1998

| | We currently bake plastic SMT parts in accordance with the IPC guidelines for moisture sensitive parts. Because we have double-sided boards that are subjected to either an aqueous or semi-aqueous cleaning process, we do a 24 hour bake at 125 deg. C after the first side of the board is reflow solder and cleaned. | | Does anyone know of any studies to indicate either of the following: | | 1. Is baking after the first side of the board is solder and cleaned really necessary if the board is processed within the time frame recommended for moisture sensittive parts? | | 2. Does vacuum baking reduce the cycle time for moisture bakeout? If yes, what vacuum ovens are recommended and what is the bakeout schedule? | | Thanks, | Pete, | Not to pry but whatcha buildin'? If that's proprietary, no big deal. The reason I ask is because when I have built stuff for the medical industry, for example, I have to follow rigid process guidelines. Sometimes, even some consumer electronics customers designate processes. My point is this; You don't need to bake after the first component population / reflow. I never did and I never had a problem. I've built about a bazillion different assemblies too so my statement is not lacking the backing. I think Wayne pretty much nailed the vacuum oven part. I used regular convection / baking ovens when trying to clear moisture from parts. You DO want to wash after your first SMT assembly operation. I assume you're running a water soluble process. Not washing would more than likely turn your flux residue into something just short of concrete if you you pass that stuff through a second reflow profile. | Cheers, | Justin Justin, You are right about cleaning after first pass. I was lost in space again for only the second time in my life, but you brought me back. We have no problem Justin. Thanks, Earl Moon

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