I somehow must have missed your E-mail in the long trail. I will attempt to answer your questions:
Here is a brief history on the problem: (my apologies for the excessive length of the e-mail in advance). Steve, can you please post these pictures on your website?
We manufacture a leaded version of the assembly using FR408 material without any issues. When the assembly went lead-free, we assembled it on FR370HR and had cracks on the BGA device. Initial x-section revealed:
1. Cracks on the PCB side near the solder/pad interface on some joints
2. Pad ruptures on some joints
3. Crack on the component side on 1 joint
4. Crack at the PCB side interface as well as pad rupture on 1 joint, here is the picture
[cid:[log in to unmask]]
All these joints were near the corner of the PCB. The FA lab concluded that the cracks were stress induced. They however also mentioned this:
“The Ni surface on the PCB-side pads is somewhat irregular. The Ni is not excessively thin, however. The presence of intermetallic at the interface shows that a “Black Pad” condition is not present. The uneven surface, however, may reflect a plating bath that is potentially unbalanced with respect to chemical concentrations”
I then started looking at if these cracks were appearing during reflow (due to warpage) and did another x-section of the board immediately after reflow (no secondary processes). There were no cracks observed in the sample. Again, however the FA lab mentioned this:
[cid:[log in to unmask]]
“Irregular Ni surface and thick P-rich layer is not typical however thick intermetallic formation suggests a strong metallurgical bond.The Ni surface on the PCB-side pads is somewhat irregular. The Ni is not excessively thin, however. The presence of intermetallic at the interface shows that a “Black Pad” condition is not present. The uneven surface, however, may reflect a plating bath that is potentially unbalanced with respect to chemical concentrations. ”
We therefore decided to look for (a stronger) alternate laminate.
We did a comparative study of laminate peel strength with FR408-SnPb temp, FR408-LF temp. FR370HR-LF Temp. we found that (even) the FR408-LF Temp was significantly stronger than FR370HR-LF combination.
[cid:[log in to unmask]]
Failure mode observed on FR406, Pb-Free process. Pad is removed from laminate, with secondary failure of intermetallic layer.
The FR406 board and FR370HR board subjected to Pb-Free processing exhibited mixed-mode failures; both cratering and IMC fracture was observed. The IMC fractures were secondary failures occurring after the pad had failed.
The cooling rate is 1.7 deg/sec. I am going to see if I can measure the P content in the P-rich layer.
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Thursday, June 19, 2008 5:13 PM
To: Kane, Amol (349); [log in to unmask]; [log in to unmask]
Subject: Re: [LF] [TN] SN100 for Reflow Application
There is something happening that you have not observer/mentioned.
You say: "LF laminate subjected to Pb-Free processing exhibited mixed-mode failures; both cratering and IMC fracture was observed. The IMC fractures were secondary failures occurring after the pad had failed."
So what is different? What is different is the much higher LF-soldering T—thus, whatever is happening is most likely related to the high T's.
What I do not understand is, how you can get IMC fractures after the pad cratering has occurred—if the SJ is no longer attached to the PCB, where are the loads coming from? When you say IMC failures, what exactly do you mean? IMC layers very rarely, if ever fail as such—they may be on one side of the separation with the other side being somethiing other than IMC.
The PCB thickness is not the issue, fixturing should avoid problems—there are other underlying root causes. How fast are you cooling from reflow?
At the end you say that "our SnPb variant of the same product...works without any issues", but earlier yoou said you got pad cratering. Which is it?
Solder Joint Reliability: Parts 1 through 4, July 17/18, Thal, Switzerland
Pb-Free Soldering Processes—Survival, Quality, Reliability, August 18, Orlando
Reliability Issues with Lead-Free Soldering Processes, September 22, Schaumburg
Failure Mode and Root Cause Analyses Reliability (Fatigue, Brittle Fracture, ENIG), September 22, Schaumburg
Gas prices getting you down? Search AOL Autos for fuel-efficient used cars.
WARNING: Export Control
This document may contain technical data within the definition of the
International Traffic in Arms Regulations (ITAR), and subject to the Export
Control Laws of the U.S. Government. Transfer of such data by any means to a
foreign person, whether in the United States or abroad, without proper export
authorization or other approval from the U.S. Department of State is
This e-mail, and any attachments, is for the sole use of the intended
recipient(s) and may contain information that is confidential and
protected from disclosure under the law. Any unauthorized review, use,
disclosure, or distribution is prohibited. If you are not the intended
recipient, please contact the sender by reply e-mail, and delete/destroy
all copies of the original message and attachments.