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January 2009

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From:
Guy Ramsey <[log in to unmask]>
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Date:
Thu, 8 Jan 2009 13:29:17 -0500
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Ever really watch a PCB go over a solder wave? 
They warp. It is a problem with BGAs on top. If they have vias in dog bones it can take the joint into reflow and cause opens and disturbed solder joints. 

Wave soldering and reflow soldering are completely different animals. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Cong He
Sent: Tuesday, January 06, 2009 8:24 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]>


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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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