Research
Impractical Stencil Aperture Designs to Enable M0201 Assembly
Effectiveness of I/O Stencil Aperture Modifications on BTC Void Reduction
Microalloyed Sn-Cu Pb-Free Solder for High Temp
Selective Reflow Rework Process
Impact of Thermal Loading on the Structural Intergrity of 3D TSV Package
Design and Fabrication of Ultra-Thin Flexible Substrate
Influence of PCB Surface Features on BGA Assembly Yield
Last Will and Testament of the BGA Void
MORE RESEARCH
Latest Industry News
Print These Electronic Circuits Directly Onto Skin
Compal increasingly asked to diversify production bases
Intel's margins tumble as customers shift to cheaper chips, shares slide 10%
From Foldable Phones to Stretchy Screens
6 Considerations for Integrating Sensors in Vehicles
Bill Gates Says Unhappy Customers Are Good for Your Business. Here's Why.
iPhone 12 review: Upgrade for the camera, not 5G
Apple's shifting supply chain creates boomtowns in rural Vietnam
MORE INDUSTRY NEWS

Evaluation of 13 Solder Pastes for High Rel



Evaluation of 13 Solder Pastes for High Rel
The authors undertook this project to choose the most robust solder paste to support the manufacture of complex high reliability products assembled at a of locations.
Analysis Lab

DOWNLOAD

Authored By:


Scott J. Anson Ph.D., PE
LeTourneau University, Longview, TX, USA

Michael McLaughlin and Abner Argueta
IEC Electronics Corporation, Newark NY, USA

Narration and Analysis:
Jim Hall
Jim Hall, ITM Consulting
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


The authors undertook this project to objectively choose the most robust solder paste available to support the manufacture of increasingly complex high reliability products, assembled at a wide variety of global locations.

This paper presents a multivariable test and analysis methodology for solder paste evaluation. The following performance characteristics were tested:
  1. Solder Balling
  2. Slump
  3. Printability
  4. Response to Pause in Printing
  5. Voiding
  6. Wetting
  7. Solder Joint Characteristics
  8. Visual Cleanliness
  9. Ionic Cleanliness
Using this methodology the authors evaluated a variety of water soluble and no-clean paste formulations for tin-lead assembly of high reliability products such as military and medical.

Test boards incorporated backward compatible SAC BGA's, bottom terminated components (QFN's) and other packages.

Formal statistical analysis tools were utilized extensively in the evaluation of test data. A weighted matrix was used to integrate all test data into a single ranking value.

Results show significant performance variations between paste formulations (not all pastes are created equal). They also indicated that the development of flux chemistry for tin-lead, water soluble pastes typically used for high reliability products has lagged behind lead-free and no-clean pastes.

This was demonstrated by lower performance in many individual tests and overall rankings.

I feel that this last finding sheds significant light on the growing interest in the seemingly incongruous practice of cleaning no-clean pastes for high reliability applications.

Summary


Solder paste is the most important material in Surface Mount Technology (SMT) assembly, and contract manufacturers of high reliability electronics face the simultaneous challenges of multisite operations combined with mission critical customer applications. It is essential to select a solder paste that performs at a high level under optimum conditions and also gives stable performance under suboptimal conditions, such as common environmental and manufacturing time fluctuations.

This study examined 63Sn/37Pb solder paste alloy with both 63Sn/37Pb and 96.5/3.0Ag/0.5Sn (SAC305) lead free components in mixed alloy reflow. Both water soluble and no-clean chemistries were evaluated. A design of experiments (DOE) approach was employed to examine solder balling and slump resistance. Additional experiments examined print quality utilizing automated Solder Paste Inspection (SPI) in terms of bridge resistance, CSP and 0201 insufficient resistance, and response to a 45 minute print pause. Assemblies were also built to check for defects, joint quality, and voiding.

The result is a holistic approach to solder paste testing that includes weighting of numerous factors to assess solder paste performance and robustness to external variation such as humidity and exposure time.

Conclusions


Evaluation of solder pastes is a complex multivariable problem. It is not sufficient for a solder paste to excel in many areas but be inadequate in one or more critical areas. Rather, an assessment of best overall must be made to determine a most effective solder paste. It is quite possible that the best solder paste overall not be the best in any one test criteria.

The variation of humidity and exposure time over each paste allowed for selection of a robust product that will run in a quantified range of operating environments. The use of the modified FMEA with weighting factors, allowed numerous criteria to be analyzed at one time. This combined with ANOVA in designed experiments and simpler low sample size experiments, provides a practical and balanced approach to selecting an optimum paste when multiple performance criteria are at play.

As a group the seven no-clean solder pastes greatly outperformed the six water soluble solder pastes in every category, but especially in slump, solder balling and printing. Discussion with the solder pasted vendors confirmed that most new flux effort goes into lead free chemistries with no-clean first, followed by water soluble. Then remaining effort goes into 63Sn/37Pb chemistries with most effort in no-cleans and then water soluble chemistries. Some vendors disclosed that any 63Sn/37Pb flux development is simply a testing of lead free chemistries in 63Sn/37Pb alloys.

It became evident that major solder paste vendors have limited experience blending 63Sn/37Pb water soluble solder paste in type 4 particle size. In order to prevent any supply chain delays, a safety stock of one standard shipment was always held in reserve at the paste manufacturer such that when a shipment is ordered and a batch does not pass stringent quality controls, the safety stock batch can still be shipped. The paste vendor's limited development effort 63Sn/37Pb water soluble sold pastes and limited experience in blending type 4 powders in this metallurgical and chemistry combination, necessitate detailed quantitative paste evaluation and acceptance criteria.

This is an result of 63Sn/37Pb water soluble type 4 being the unique domain of high reliability manufactures that produce mission critical electronics where weight limitations result in smaller components and finer paste particle sizes. High reliability electronics manufacturers find themselves in the domain of mission critical products that are built with 63Sn/37 Pb for maximum reliability, and yet these same solder pastes are not the research or product focus of majorsolder paste manufactures. There seems to be a vacant niche for a solder paste manufacturer to differentiate themselves with a focus on 63Sn/37Pb water soluble solder pastes in type 4 for the high reliability market sector.

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
Causes of Blowholes
Tips When Moving a Reflow Oven
Assembling Boards with BGAs on Both Sides
Larger Stencil Apertures and Type 4 Paste
5 vs 8-Zone Ovens
Component Moisture Question?
BGA Components and Coplanarity
How To Verify Cleanliness After Rework and Prior to Re-coating?
MORE BOARD TALK
Ask the Experts
Initial Screen Print Test Board
HASL Surface Finish and Coplanarity
Legend Marking Discoloration
Cleanliness Testing
Stencil Cleaning Frequency
Exposed Copper Risk
Spotting After DI Water Cleaning
ESD Grounding - 1 Meg Ohm Resistor
MORE ASK THE EXPERTS