Research
Implementing Robust Bead Probe Test Processes
Via-In Pad Plated Over (VIPPO) Design Considerations
Screening of Lower Melting Point Pb-Free Alloys
Condensation Testing - A New Approach
Case Study for Improving the PCB Print Process Using Factory Data
DFX on High Density Assemblies
Improved SMT and BLR of 0.35 mm Pitch Wafer Level Packages
Wetting and Solidification of Pure Tin on Polycrystalline Intermetallic Substrates
MORE RESEARCH
Latest Industry News
MacBook, iPad Production Delayed as Supply Crunch Hits Apple
Microsoft unveils liquid cooling solution for datacenters
A Theory of (Almost) Everything
Toyota unveils new models in advanced driver-assist technology push
Tech Giants Enter Their Chips in the Race for Self-driving Cars
What will self-driving trucks mean for truck drivers?
4 Great Things That Happened When We Went Remote
iPhone 13 is going to be worth the wait: All the major upgrades we're looking forward to
MORE INDUSTRY NEWS

Achieving Solder Reliability for LGA Ceramic Image Sensors



Achieving Solder Reliability for LGA Ceramic Image Sensors
The stencil aperture design has more influence on overcoming CTE mismatch between the ceramic LGA and the FR4 PCB than the reflow oven temperature profile cool down slope.
Analysis Lab

DOWNLOAD

Authored By:


Lynda Pelley, Teledyne DALSA Inc., Waterloo, ON, Canada
Hugo Gallegos, SMTC Inc., Chihuahua, Mexico

Summary


New product introduction at an Original Equipment Manufacturer (OEM) typically includes reliability testing in the form of thermal cycling. Samplings from the engineering models of a new product line were consistently failing the optical testing following the reliability testing. Cross sectioning revealed stress fractures in the solder joints of the LGA ceramic image sensor component. As the image sensor dominates one side of the PCA, the root cause appeared to be Coefficient of Thermal Expansion (CTE) mismatch between the FR4 material of the PCA and the ceramic body of the image sensor. As this new product line was to be high volume, the goal was to avoid introducing an additional component under fill process step and resolve reliability issues through basic SMT assembly processes at the Contract Manufacturer (CM).

A joint OEM-CM Tiger Team explored the contributions of both the solder paste volume and the reflow temperature profile to the robustness of the LGA solder joints. The stencil design aspect included the shape of the aperture and the resulting volume of solder paste deposited. The solder paste volume affects the standoff of the component from the PCA. The sensor manufacturer datasheet for the oven reflow profile allowed a range of temperature values, the most significant being the temperature cool down slope. The cool down slope affects the granularity of the solder joint. The CM’s team performed a DOE maximizing and minimizing both the solder paste volume and the temperature profile cool down slope combinations.

The DOE results trended towards higher paste volume and steeper temperature cool down slope. The risk of solder bridging using the higher paste volume was minimal, but the steeper temperature cool down slope produced unacceptable process voids. Therefore, the choice for optimal SMT process condition was a combination of high solder volume and lower temperature cool down slope. Passing results for optical testing post reliability testing, followed by cross section, validated this optimization.

Conclusions


Based on the DOE, the stencil aperture design has more influence on overcoming CTE mismatch between the ceramic LGA and the FR4 PCB than the reflow oven temperature profile cool down slope. Considering the results of the DOE and the identified risks with each approach, the overprinted square stencil aperture and the lower temperature cool down slope 1.8°C/second was chosen for the process design. The NPI was successful. Post DOE and engineering trials, there have not been any further CTE mismatch issues with the regular production of this product.

Initially Published in the SMTA Proceedings

Comments

No comments have been submitted to date.

Submit A Comment


Comments are reviewed prior to posting. You must include your full name to have your comments posted. We will not post your email address.

Your Name


Your Company
Your E-mail


Your Country
Your Comments



Board Talk
Solder Paste Volume for BGA Rework
Reflow For Rigid Flex
Delay Before Cleaning Partial Assemblies
Problems With Starved "J" Lead Joints
Solder Paste Transfer Efficiency - What/Why
Can a CTE Mismatch Cause Reliability Problems?
Going Beyond Your Solder Paste Work Life
Issues Mixing Silicone and Acrylic Conformal Coatings
MORE BOARD TALK
Ask the Experts
Bottom Terminated Components and Vias
Is Solder Mask Considered an Insulator
Reduce Glare During Assembly
BGA BAll Sheer Testing
Components Jumping Around During Reflow
Floor Life of MSD Parts
QFN Open Solder Joints
Delamination Causing Scrap
MORE ASK THE EXPERTS