TECHNET Archives

May 2014

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Thu, 1 May 2014 11:59:50 +0000
Content-Type:
text/plain
Parts/Attachments:
text/plain (172 lines)
It is very easy to determine the maximum saturated moisture content and the minimum amount of time to bake a PWB to ensure it contains less than 20% of its maximum saturated moisture content. If it contains less than 20% of its saturated moisture content, you will generally not have any issues with delamination, z-axis expansion, or CAF. Here are the general steps, refer to the details in IPC 1601.

1. Place the board in a condensation chamber for a minimum of 24 hours
2. Take it out, blow off any surface moisture, and immediately weigh the sample on a precision balance (mettler-toledo or equivalent). This is your saturated (maximum moisture content) weight.
3. Place it in an oven set at a temperature that is convenient for you. 105 deg. C is an excellent temperature for baking boards. I would not go higher. Never mind vacuum baking, it is not significantly faster.
4. Every 30 minutes, take the PWB out of the 105 deg. Oven and weigh it on the balance. Record the weight and time and quickly place the pwb back in the oven. Do this for at least a 24-hour period if you can.
5. Plot the data on a simple time/weight curve. An Excel graph does this very well.
6. Determine how many hours it takes to get from the saturated weight to a level of moisture that is 20% of that saturated weight. Typically, the reduction in weight after that point is very insignificant over time. 

That number of hours is the optimum bake time for that particular PWB part number. Any shorter, and you may end up with boards that are higher than 20% of saturated weight. Any longer, you are not reducing the water weight by much, and you may be reducing solderability (a small amount) for no good reason. As I said, you do not typically see any issues when processing PWBs that are at 20% or less of saturated moisture content. The exceptions are if the PWB itself has poor prepreg, insufficient laminate resins, resin recession behind the hole wall from dull bits, etc. Other issues can be caused by an improper profile such as an extremely fast ramp-up rate (more than 1 deg. C per second), too high of a maximum temperature during reflow (too long of a time above solidus temperature), or other process issues unrelated to PWB moisture content.

In a nutshell, your goal is to establish a bake time for a given PWB part number to ensure that at least 80% of the maximum possible moisture content (worst-case condition) is removed. This amount of bake time (at 105 deg. C) will vary depending on the number of layers, the amount of copper, etc.  Different IPC 4101 slash types will have different optimum bake times.
The lower the bake temperature, the longer it will need to bake to drive out the moisture. Moisture turns to steam at 100 deg. C, so that is why I choose 105 deg. C, just above that point.

Having said all that, now for the other side of the coin.

Once you have established the optimum bake time for a given PWB part number, don't shoot yourself in the foot by not having groundrules for how the PWB is handled AFTER baking.
It does no good to bake the board dry if you are going to then stack them on a shelf somewhere for more than 4 or 5 hours before you even process them through reflow. PWBs can quickly absorb at least 80% of their maximum saturated water content in just a few hours.

So, after baking, the standard method of handling them is to:
1. Print solder paste, pick and place the components, and get them through reflow within 2 or 3 hours.
2. If they cannot be processed through reflow immediately, then place them in a desiccator box (drybox) immediately after baking, until ready to be processed. My preference is Dr Storage. Contact Kris Meacham at [log in to unmask] He can help you pick the best size and type to meet your needs. Desiccation dry boxes are not real expensive, typically less than $6k for a good sized one.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of SALA GABRIELE
Sent: Wednesday, April 30, 2014 4:35 PM
To: [log in to unmask]
Subject: [TN] R: [TN] RES: [TN] PCB Humidity

Juliano, you're right,

for PCB the Sensitivity Level are not available. Too many variables could influence the M-Sensitivity. I found literature attributing MSL-3 at least.
It could be, but how we are sure about the kind of laminate, amountof prepreg, % of resin, % of fibreglass or copper, PCB size and thickness, number of layers, and so on. All PCB/PWB are different among them. 
Then establish the MAMC( Maximum Acceptable Moisture Content) acceptable for those PCB. 
Best way to define the MAMC is indicated in the IPC-TM-650 2.6.28 attached to IPC-1601 ( or downloadable from IPC for free). 
Not so easy test to perform if Laminate Witness Coupons ( see IPC-1601 4.2.5
) are not available on your PCB (Panel)
I know, you now are in front a practical problem to solve and not have time to spend on theories, but that is the right way to understand the technique.
 
Other points, which kind of packaging is protecting your PCB ?  Dray Pack ?
Real Dry Pack ? or simply thermal shrinkable PE or PET with a very weak material thickness on corner of package ? bottom PE pluryball?  etc...

Also, those PCB have to withstand SnPb or Pb Free Reflow temperature ? More critical situation in case of moisture entrapped in PCB when Pb Free Reflow.

Be careful, by experience, often HIC dos not tell you the real moisture content inside PCBs. It exist difference in % moisture absorbed, according to if PCB is on top of pile, inside the middle or bottom of packaging (pile). By following the TM-650 2.6.28 I discovered interesting moisture absorption behaviours among PCB packaged inside same pack.
HIC is a good help but some time gives misleading indications, be careful when interpreting it ( by experience). 

Do not forget also there are fake HICs running around.

Check the fabrication Data Code of your PCB, if not so old, don't worry so much . Just to stay quite, if you have an oven available, do a light bake, Max 125°C  8h or less. Pile-up  the PCB inside the oven max 25 mm tall.
Once bake is completed, leave PCB in the oven until room temperature
(almost) will be reached, then remove and use (reflow) very quickly, as soon as possible. Remember, no need to totally dry-off the PCB, just keep the MAMC at the appropriate %, considering also if Pb o PB free reflow temperature those PCBs have to withstand 

If PCB  fabrication Data Code is too old, I suggest you to follow TM
650-2.6.28 before to guess if bake need to be done or not. Some time is better not perform PCB bake then perform it in a wrong way. 

etc...

Gabriele



-----Messaggio originale-----
Da: TechNet [mailto:[log in to unmask]] Per conto di Datacom - Juliano Ribeiro
Inviato: mercoledì 30 aprile 2014 22.10
A: [log in to unmask]
Oggetto: [TN] RES: [TN] PCB Humidity

Thank you Gabriele for your answer, but the IPC1601 don't have an information about what are the maximum level Humidity? Or I didn't find this information.
For example, the components according to thickness are Level 3 or 4 or....
and the PCB Is there table about this?
We need to register in my incoming to verify the HIC, but what's acceptable humidity?

Thank you

_____________________________
Juliano Bettim Ribeiro
DATACOM
ENGENHARIA DE PROCESSOS
Rua América Nº 1000 - Eldorado do Sul - RS CEP: 92990-000 
+55 (51) 8446-2135
+55 (51) 3933-3000
Ramal: 3877
[log in to unmask]
www.datacom.ind.br

-----Mensagem original-----
De: SALA GABRIELE [mailto:[log in to unmask]] Enviada em: quarta-feira, 30 de abril de 2014 16:10
Para: 'TechNet E-Mail Forum'; 'Datacom - Juliano Ribeiro'
Assunto: R: [TN] PCB Humidity

Hi Juliano,

this standard gives worthy guidelines:

IPC-1601  Printed Board Handling and Storage Guidelines.

http://www.ipc.org/TOC/IPC-1601.pdf

Gabriele


-----Messaggio originale-----
Da: TechNet [mailto:[log in to unmask]] Per conto di Datacom - Juliano Ribeiro
Inviato: mercoledì 30 aprile 2014 19.57
A: [log in to unmask]
Oggetto: [TN] PCB Humidity

Hi to all,

We received a PCB lot with HIC more than 10% but less than 60%.

What is IPC/JEDEC or others specifications to Humidity limits to PCB?

Is it good or no good PCB?

 

Thank you

_____________________________

Juliano Bettim Ribeiro

DATACOM 

ENGENHARIA DE PROCESSOS
Rua América Nº 1000 - Eldorado do Sul - RS CEP: 92990-000 
+55 (51) 8446-2135

+55 (51) 3933-3000 

Ramal: 3877
 <mailto:[log in to unmask]> [log in to unmask] 
 <http://www.datacom.ind.br/> www.datacom.ind.br

 


______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________


---
Questa e-mail è priva di virus e malware perché è attiva la protezione
avast! Antivirus.
http://www.avast.com


______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________


---
Questa e-mail è priva di virus e malware perché è attiva la protezione avast! Antivirus.
http://www.avast.com

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

ATOM RSS1 RSS2