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January 2009

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 7 Jan 2009 07:24:45 -0600
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You preheat the topside. 



-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of Cong He

Sent: Tuesday, January 06, 2009 7:24 PM

To: [log in to unmask]

Subject: [TN] 答复: Re: [TN] Question about bottom heaters of reflow oven



Hi !



As you said, single side heater may cause wraping.



Then I want to know how you avoid that happen when you use wave soldering?









Hecong



Ruijie network











Werner engelmaier <[log in to unmask]>

发件人:  TechNet <[log in to unmask]>

2009-01-07 05:12

请答复 给

TechNet E-Mail Forum <[log in to unmask]>; 请答复 给

Werner engelmaier <[log in to unmask]>





收件人

[log in to unmask]

抄送



主题

Re: [TN] Question about bottom heaters of reflow oven













 Hi Dean,

Some wishful thinking for the New Year---Freudian slip at 140 C--p-i am 

sure you meant 240 C.



werner





 





 



-----Original Message-----

From: Stadem, Richard D. <[log in to unmask]>

To: [log in to unmask]

Sent: Tue, 6 Jan 2009 2:34 pm

Subject: Re: [TN] Question about bottom heaters of reflow oven





















There are lots of reasons to use both sets of heaters.



The first one that comes to mind is the fact that with only the topside

heaters going, you may create a large temperature gradient (Delta T)

between the top side of the board and the bottom. Even if you are only

soldering the topside, the temperature gradient can cause the topside of

the pwb to expand much more rapidly than the bottomside. This can lead

to a number of PWB defects, such as sheared vias and internal trace

pullaway, as well as warpage and delamination caused by the internal

stresses due to the large difference of CTE between the top and bottom

of the board.

 

Secondly, if there are BGAs or other large components such as QFN

packages on the board, the package bodies do a very effective job of

insulating the heat from the board. To effectively solder these kinds of

packages (and really all components) you need to have some type of heat

conducted through the PWB pads, through the boards itself. This soaking

of the entire board typically does not hurt the PWB, provided the reflow

profile parameters are appropriate for the board type. Subjecting it to

large temperature differentials does much more harm.



Third, you can actually melt solder paste and have it "wet" over the

pads, but the board itself needs to be at least 140 C or thereabouts to

effectively dissolve some of the pad metallization into the solder in

order to form a good intermetallic bond with the solder. Running a cold

board through the oven inhibits the ability for this dissolution to take

place. The melted solder paste might "look" good enough to meet IPC 610

specs, but the reliability of the SJs will be suspect, at best.

 



-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of Eli Sarig

Sent: Tuesday, January 06, 2009 1:16 PM

To: [log in to unmask]

Subject: [TN] Question about bottom heaters of reflow oven



Hi all,



My question is related to the necessity of down heaters in the reflow

oven.

When we solder double sided boards, we *can* create a suitable reflow

profile using *only* the top heaters of the oven.

In that case, the bottom heaters remain at room temperature. We measure

almost the same temperature on top and bottom of the board.

Are there any limitations or problems that might happen using this kind

of process?



Thanks,



Eli.



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