Thank-you John. These are all very good pointers.
 
I do have the solder alloy composition now ---Sn/Ag(3.0%)/Cu(0.5) -
Cream Solder.  They have a low Tg board less than 150 C, so I
recommended , they try a higher Tg PCB.
 
I have posted the reflow profile on the
http://ipc-technet.groupsite.com/main/summary
<http://ipc-technet.groupsite.com/main/summary>  in the files tab.
 
Any comments/feedback will be appreciated.
 
Thank-you, Mumtaz

 

Mumtaz Bora

Senior Packaging Engineer

Peregrine Semiconductor

9380 Carroll Park Drive

San Diego, CA 92121

858-795-0112-direct

858-731-9499 -fax

[log in to unmask]

 

 

________________________________

From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: Monday, December 06, 2010 1:03 PM
To: TechNet E-Mail Forum; Mumtaz Bora
Subject: Re: [TN] QFN package crack post Reflow



Hello Mumtaz

1. Looking at the pictures I see that it appears the damage was caught
not in SMT inspection but after wave solder and the cleaning process. I
don't see any No-Clean residue so it looks like water soluble flux was
used.

 - I just realized these are customer returns. I don't know how thick
the board is. The test operators could be causing the stress if
connnecting to the coax connector and the board is not secured in a
chassis. The only other step after final inspection if packing in the
shippping area if the boards are shipped individually and not in chassis
and not secured to a mother board. They may not realize how careful they
have to be in handling them. The other end is the customer's receivers.
You may have to audit the customer when the board arrives and they are
unpacking and how they storing them. 

 

2. Looking at the direction of the cracks towards the coax connector
side of the board I wonder if they are using those slotted board racks
rather than placing the boards down flat on a tray lined with an ESD
mat. It would be important to check the QFN after reflow and again if
the boards are cleaned after SMT to get the flux off within the time
limit or to provide clean boards to pre-wave assembly. If the QFN are
good to this point then the problem is in the following process steps
I've included below.  

 

3. Another possibility is if there is some stress in the area of the QFN
when the coax connectors are inserted due to hole size or extra solder
on the leads or holes. With the added weight of the coax connectors
slotted board racks should not be used.

 

4. Another stress issue may be if spring loaded clamps are used to hold
the coax connectors flush to the PCB. If the clamps are swung over and
allowed to snap down, then stress could cause the problem. Trainig will
correct the problem at wave solder.

 

5. Another stress point is in the handling of the boards and the
orientation of the boards when they go in the cleaner. The heavy side
must be toward the leading edge when put on the cleaner's conveyor belt.
If the light end is leading then the board will be bouncing all the way
through the cleaner and when the hot air knives dry the board. If the
board is directly on the cleaning belt and not on specially designed
nitril/carbon cleaning mats then many of the SMT parts will be stressed
and the bottom ones knocked off when operators slide them out the exit
end of the wire conveyor belt.

 See attachment. The attachment may help you if not with this issue,
then with some other process improvements.

John Goulet

MFG Engineer


----- Original Message -----
From: "Mumtaz Bora" <[log in to unmask]>
To: [log in to unmask]
Sent: Sunday, December 5, 2010 2:00:27 AM
Subject: [TN] QFN package crack post Reflow

 
Hello Technetters,
 
Received a customer return showing QFN package cracking after double
sided reflow on a board with coax connectors near the package.  Board
shows a lot of thermal mass on one side. Do not have the details of
solder material and reflow  profile. I have posted a .PDF file of images
on the following link. Please go to the file tab , not photos. Your
comments/feedback will be appreciated. 
 
 
http://ipc-technet.groupsite.com/main/summary
<http://ipc-technet.groupsite.com/main/summary> 
 
 
 
Thank-you, Mumtaz
 

Mumtaz Bora

Senior Packaging Engineer

Peregrine Semiconductor

9380 Carroll Park Drive

San Diego, CA 92121

858-795-0112-direct

858-731-9499 -fax

[log in to unmask]

 

 

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