-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueComing to Terms With AI
In this issue, we examine the profound effect artificial intelligence and machine learning are having on manufacturing and business processes. We follow technology, innovation, and money as automation becomes the new key indicator of growth in our industry.
Box Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Acceptance Testing Of Low-Ag Reflow Solder Alloys (Part 1)
August 12, 2015 | K. Troxel, A. Allen, E.E. Benedetto, R. Joshi, Hewlett-Packard Co.Estimated reading time: 9 minutes
There were two manufacturing design of experiments (DoEs) as part of each SMT alloy evaluation. Both consisted of building boards with a low-Ag SMT alloy and a SAC305 control at defined process parameters (with multiple peak reflow temperatures and times above liquidus, TAL). Small test boards were selected for each DoE so that they could be processed with very small temperature deltas across the boards, eliminating this variable. This gave confidence in the DoE results on whether an alloy was able to form acceptable solder joints at each specified temperature and TAL.
BGA solder joints were evaluated for two different sizes of BGA component (see test board in Figure 1). In this DoE, three parameters were evaluated: BGA solder joint formation (in cross-section) per IPC-A-610, intermetallic compound (IMC) thickness, and Cu dissolution.
Figure 1: BGA Manufacturing DoE Test Board. Boxed locations (U4, U9, PCB pad for U11) are thermocouple locations.
The second DoE evaluated numerous leaded components (see test board in Figure 2). In this DoE, leaded solder joint formation per IPC-A-610 was assessed for the low-Ag alloy.
For the low-Ag alloy to be feasible for use in general PCA assembly, it must not dissolve excessive amounts of Cu, it must not form very thick IMCs that might be prone to fracture under mechanical stresses, and it must form acceptable solder joints within the OEM’s PCA processing window. The first two requirements (Cu dissolution, IMC thickness) were evaluated at a single process condition, 250 °C and 120 seconds TAL, processed three times. This process condition was selected as representative of the PCA process conditions of a complex product board that is reworked twice. This would be the worst-case conditions, from an IMC growth and Cu dissolution standpoint. The last requirement, forming acceptable solder joints, was the basis for the wide range of TALs and peak reflow temperatures in both DoEs; ranges beyond what would be considered a normal process window, but necessary to determine the processing cliffs where the low-Ag alloy fails to form acceptable solder joints. An example of the manufacturing DoE process conditions is in Table 2.
Figure 2: Leaded component DoE test board. Numbered components are to be populated (1-5 to be cross-sectioned). Ref: SMTA Saber
Of the 15 process conditions assembled, a subset (green boxes) was selected for cross-sectioning and inspection to determine BGA solder joint formation per IPC-A-610, IMC thickness, and Cu dissolution. If questions arose on the data, additional process conditions (yellow boxes) may be evaluated.
Wetting balance was evaluated for the low-Ag alloys (compared to SAC305) by following industry test method J-STD-003. A sample size of 10 coupons, per alloy, was used. A standard activated rosin flux #2 for Pb-free alloys was specified to keep different vendor test results consistent. The test boards were preconditioned for 8 hours at 72 °C/85% RH, followed by 1 hour at 105 °C. For the wetting balance test, the coupons were immersed in molten solder, 45° incident to the solder pot, to a depth of 0.4 mm at 2 mm/second.
Table 2: Example process conditions for manufacturing DoE.
Reliability Tests
The reliability tests included accelerated thermal cycling (ATC) and mechanical shock testing. Both these tests used BGAs processed at a single process condition that formed acceptable solder joints (per IPC-A-610). The BGA Manufacturing DoE Test Board (see Figure 1) was used for both reliability tests, populated with the eight smaller CABGA-192 components.
Mechanical shock testing was performed per JEDEC standard JESD22-B111 (test condition B, 1500 G, 0.5 ms duration, half-sine pulse). The controls were Sn-37Pb and SAC405. For all alloys tested, the solder ball and SMT paste were the same alloy (no mixing of solder compositions was chosen, since a low-Ag alloy ball with SAC305 paste should have intermediate performance between a low-Ag alloy and a SAC305 solder joint). For each alloy, the mechanical shock performance was evaluated on Cu OSP and electrolytic Ni/Au PCB surface finishes. Resistance was monitored in-situ, Weibull failure plots generated, and failure modes verified with selective failure analysis after test completion.
ATC testing was executed per IPC-9701 (test condition 1) to 6000 thermal cycles. The controls were Sn-37Pb and SAC305. The test board had a Cu OSP surface finish. For all alloys tested, the solder ball and SMT paste were the same alloy, for the same reasons discussed above for mechanical shock testing. Resistance was monitored in-situ, Weibull failure plots generated, and failure modes verified with selective failure analysis after test completion.
Bulk Solder Material Properties
Bulk solder material properties evaluated included melting behavior (DSC or DTA curves per NIST SP 960-15), stress-strain curves (with a specified sample geometry per ASTM E8-04), elastic modulus (per ASTM 1875-00), coefficient of thermal expansion (per ASTM E831-06), density, hardness (per ASTM E92-82), electrical conductivity (per ASTM B193-02), and thermal conductivity.
In Part 2 of this two-part article series, the authors will discuss their test results as well as observed defects.
Editor's Note: This paper has been published in the technical proceedings of IPC APEX EXPO.
Page 2 of 2Suggested Items
Indium Experts to Present on High-Temperature, Lead-Free Solder Paste and High Reliability Liquid Metal Alloys Poster at ECTC
05/16/2024 | Indium CorporationIndium Corporation Research Associate Kyle Aserian will deliver a presentation at the 74th Electronic Components and Technology Conference (ECTC) on May 31, in Denver, Colorado.
Indium Experts to Present at Electronics in Harsh Environments SMTA Conference
05/13/2024 | Indium Corporationndium Corporation Technical Manager for Europe, Africa, and the Middle East, Karthik Vijay, will deliver a technical presentation and Indium Corporation Senior Technologist, Dr. Ronald Lasky, will deliver both a workshop and technical presentation at the Electronics in Harsh Environments SMTA Conference on May 14-16 in Copenhagen, Denmark.
Connect the Dots: Designing for Reality—The Pre-Manufacturing Process
05/08/2024 | Matt Stevenson -- Column: Connect the DotsI have been working with Nolan Johnson on a podcast series about designing PCBs for the reality of manufacturing. By sharing lessons learned over a long career in the PCB industry, we hope to shorten learning curves and help designers produce better boards with less hassle and rework. Episode 2 deals with the electronic pre-manufacturing process. Moving from CAD (computer-aided design) to CAM (computer-aided manufacturing) is a key step in PCB manufacturing. CAM turns digital designs into instructions that machines can use to actually build the PCB.
AIM Solder Signs Shinil Fl Ltd. as New Distributor for Korea
05/08/2024 | AIM SolderAIM Solder, a leading global manufacturer of solder assembly materials for the electronics industry, is pleased to announce a new distribution partnership with Shinil Fl Ltd., a prominent supplier of technological solutions in the SMT and semiconductor sectors.
Indium Corporation to Showcase HIA Materials at ECTC
05/07/2024 | Indium CorporationAs an industry leader in innovative materials solutions for semiconductor packaging and assembly, Indium Corporation® will feature its advanced products designed to meet the evolving challenges of heterogeneous integration and assembly (HIA) and fine-pitch system-in-package (SiP) applications at the 74th Electronic Components and Technology Conference (ECTC), May 28‒31, in Denver, Colorado.