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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Werner Engelmaier <[log in to unmask]>
Date:
Wed, 31 Mar 2010 16:01:01 -0400
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 Hi Amol,
Not having a heel fillet can be a problem—but not likely for a fine-pitch QFP.
Your causes of not having a heel fillet could be (1) there is no soldering pad at the heel bend (you saying you have near-vertical leads could be a clue); (2) your peak reflow temperature is too low (the component die is a heat sink and the leads are quite a bit cooler than the PCB pad); (3) not enough solder paste.
Werner

 


 

 

-----Original Message-----
From: Kane, Amol (349) <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, Mar 31, 2010 3:24 pm
Subject: [TN] Lead forming angle and solder joint formation


I have had a very similar as Bev issue come up here, only in this case it has to 
do with the absence of a heel fillet on a completely plated part. My question is 
this: Does the lead forming angle play a role in whether or not a heel fillet 
forms when a part is soldered to the pad?

We have a fine pitch QFP device (19 mil pitch) that we are soldering at SMT. Our 
quality department is rejecting the boards because of a absence of a visible 
heel fillet per IPC 610D class 3. When I look at an unsoldered part under the 
scope, the angle between the foot and the leg is damn close to 90 degrees!

We are using a 5 mil thick stencil with 1 mil reduction on the width and 1:1 in 
the length, resulting in a 2-3 mil actual solder height (after the flux has 
blown off). Has anyone seen a scenario that prevents the solder from wicking up 
the foot to the leg forming a heel fillet because of the forming angle? I am 
also looking to see if a possible contamination issue on the part can result in 
this phenomenon.


Thanks, 
Amol









-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Tuesday, March 30, 2010 9:24 PM
To: [log in to unmask]
Subject: Re: [TN] Need some opinions

Dave,

A distinction has to be made between failing the SMT process and failing the
product.  Obviously the issue has little to do with screen printing,
placement or reflow - all other parts have the proper amount of solder
paste, are placed properly, have good looking joints and intermetallic of an
appropriate thickness is there, even on the leaded component in question
(where there is an actual joint!). So the process is in control.

 

With regards to the two other component types, I have some comments.  SOIC
sheared toes have been a pain for quite some time. However, we already know
that toe fillets only provide 5% of joint strength at most and industry
available reliability data certainly shows that absence of toe fillets on
leaded components is not something to be overly concerned about.  OK, let's
discuss LGAs and QFNs with sawn vertical surfaces.

1)       The easiest way to avoid bulbous joints with these components is
tight control of pad size and the amount of solder paste deposited.

2)       If negative wetting angles are produced, they are not pointed in
the direction where it can do the most damage.  They point straight down
into the bulk of the solder.  Negative wetting angles at the heel fillet of
a leaded device are pointing right down or close to the intermetallic
demarcation line.

3)       We have never seen a bulbous joint of a LGA or QFN on a rigid board
cause a failure during thermal cycling, drop or tumble.

4)       However, we do not allow bulbous joints on these devices on flex
circuits.  I'll let everyone think about that one.  Maybe we are over
cautious, but.

 

Bev

RIM

 

  _____  

From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: Tuesday, March 30, 2010 5:30 PM
To: TechNet E-Mail Forum; Bev Christian
Cc: [log in to unmask]
Subject: Re: [TN] Need some opinions

 


Hi Bev - I wouldn't "fail" the soldering process or the solder joint if I
can show the reason is the lack of plating "by design" as you described.
However, as other folks have pointed out, there are other functional issues
having a SMT component lead sufficiently plated which need to be considered.
The industry is already starting to ask the same questions in terms of have
sheared toes (SOIC type components) or sawed surfaces (QFN type components)
but those cases are not as severe as your case. 

Dave Hillman 
Rockwell Collins 
[log in to unmask] 





Bev Christian <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]> 

03/25/2010 08:08 PM 


Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Bev Christian <[log in to unmask]>


To

[log in to unmask] 


cc

 


Subject

[TN] Need some opinions

 


 

 




Technetters,
It seems that more and more component suppliers are not plating their leads
all the way up to the component body. In fact some are getting downright
skimpy and only plating to the bottom bend, never mind the upper one. If you
put down a lot of solder paste you end up with a bulbous joint, a negative
wetting angle and a stress concentration point. If you put down less paste
and have a co-planarity issue, then you have the potential for opens.  The
other option of course is getting the supplier to plate higher up the leads.

Question: would you consider the condition of the formation of the negative
wetting angles a failure during process qualification?

Bev
RIM


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