I believe that the question is asked because in some instances there may be one board that has two choices remove all the components and start all over again or hold the board till one more fine pitch flat pack or BGA is located to finish the last board. I have been there and done that. Some pastes are very forgiving and some give one choice, start all over again. There are many other reasons why a board(s) are not processed promptly.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
Sent: Friday, September 29, 2006 7:47 PM
To: [log in to unmask]
Subject: [TN] NTC RE: [TN] Solder paste question
Recipe for a soufflé omelette.
Take 3 eggs separate the whites, beat whites until fluffy, add yolks, mix briefly into fluffed whites, pour into a hot pan coated with butter for 30 to 60 seconds. Place under broiler to allow top to seal and then the omellette will naturally rise - do not over cook the top, but rather transfer back to hot plate to cook to taste.
Recipe for SMT
Paste, check, place components within 10 minutes, reflow within 10 minutes after that..........8-)
Don't ask "how long can it remain there" but rather change the process to minimize the time if you want perfect results.
John
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
Sent: Friday, September 29, 2006 4:31 PM
To: [log in to unmask]
Subject: Re: [TN] Solder paste question
After printing the solder paste is vulnerable to cold slump as noted, also the surface to volume ratio of the printed paste deposits is high so egress of solvents and ingress of moisture is an issue resulting in loss of tack, poor wetting and so on.
The actual times for these things to happen will vary according to paste type , supplier formulations and environment.
IPC tests are conducted in standardised conditions which may not be representative of actual factory floor conditions, they should theorem be treated more as comparitive guides rather than absolute facts.
If you need to batch boards after printing before placing then you need a constant temperature low humidity low draft environment. In an ideal world you will have a nitrogen flushed cabinet to flat rack them in. . At the other end of the scale you could just put them flat in Tupperware type containers, maybe with a silica desiccant.
Regards
Mike Fenner
Indium Corporation
T: + 44 1908 580 400
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Wednesday, September 27, 2006 9:26 PM
To: [log in to unmask]
Subject: Re: [TN] Solder paste question
A little bit of tack testing and plenty of information on slump test is covered in J-STD-005 and J-STD-006, and in the TMs in 650 called out within those documents, but probably not enough regarding the application of the pastes.
The application is important as well, ie, if you are running a high-speed turret-type placer the tackiness and slump qualities are much more critical than if you are running a gantry style placer. And if you are dealing with pitches of .8mm or less, again the ability of the paste to form a good, clean brick without slumping is again very critical.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Wednesday, September 27, 2006 2:12 PM
To: [log in to unmask]
Subject: Re: [TN] Solder paste question
Thanks for the correction Steve. I have seen the equipment that does the Sipad process and talked with one of the fellows that is pioneering that process in the USA at length. The main point, some pastes loose their tackiness in about an hour and some last as Steve said much longer. A test should be made to see how long does a particular formulation keeps its tackiness and then stay within the time. Of course the test board should be fully inspected for good solder joints.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Wednesday, September 27, 2006 2:16 PM
To: [log in to unmask]
Subject: Re: [TN] Solder paste question
Solder Pastes that are tacky for weeks? I don't think so.
SiPad is not a solder paste but a solid solder deposit that has been flattened and then a tacky flux applied over that. Then the board is covered with a slip sheet to keep the tacky flux reasonably fresh.
-Steve Gregory-
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Wednesday, September 27, 2006 1:00 PM
To: [log in to unmask]
Subject: Re: [TN] Solder paste question
Joel:
Some pastes can be tacky for weeks (Sipad) and some will loose their tackiness in an hour and parts start sliding over the solder balls, at which time parts need to be taken off the board, board cleaned and part placement process started again. It all depends on the solder paste formulation. Placing a board on its side will risk parts sliding down, not recommended, unless you are using Sipad, very tacky firm paste.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Wednesday, September 27, 2006 11:27 AM
To: [log in to unmask]
Subject: Re: [TN] Solder paste question
Hi Joel!
First off, I'm not aware of any "Industry Standard", because it's all going to depend on the solder paste. All solder paste "slumps" to one degree or another, depending on the formulation, environment, etc..
Personally, I wouldn't like to see a printed board set for more than an hour or two before it was placed if you can help it...especially if there's any fine pitch or BGA's on the board.
Saying that, there's been some unusual circumstances where I've had to hand place boards and they've sat for eight hours before I had all the components placed and the boards reflowed. Once or twice in another life, it's taken more than a day to finish hand placing a board (I don't ever want to do that again!) and getting it in the oven. Did the boards reflow okay? Kinda'...there WAS just a little touch-up needed! (GRIN)
When you say sideways on a tray, are you talking about tilt-racks?
Because if you are, I would say that isn't a good idea. Gravity is only going to exacerbate any slump issues that the paste experiences.
-Steve Gregory-
-----Original Message-----
From: TechNet [mailto:[log in to unmask] <mailto:[log in to unmask]> ] On Behalf Of Joel Alexander
Sent: Wednesday, September 27, 2006 10:06 AM
To: [log in to unmask]
Subject: [TN] Solder paste question
Is there any industry standard on how long a printed board can sit prior to reflow? Also if a printed board is placed sideways on a tray, can this cause print quality issues?
Joel Alexander
APSCO Quality Assurance Manager
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