-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox 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.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
The Impact of Via and Pad Design on QFN Assembly
January 25, 2017 | David Geiger, Anwar Mohammed and Jennifer Nguyen, FlexEstimated reading time: 3 minutes
ABSTRACT
Quad flat no-lead (QFN) packages have become very popular in the industry and are widely used in many products. These packages have different size and pin counts, but they have a common feature: a thermal pad at the bottom of the device. The thermal pad of the leadless QFN provides efficient heat dissipation from the component to PCB. In many cases, a thermal via array under the component is used to conduct heat away from the device. However, thermal vias can create more voids or result in solder protrusion onto the secondary side.
This paper discusses our study on the impact of via size and via design on QFN voiding and solder protrusion. Does a small via prevent the solder to flow to the other side? How should the via be designed? Which via type will have less of a voiding issue? A comprehensive experiment was designed to try to answer these questions. Different QFN types, via design, via sizes, via pitches and stencil design were studied using three different board thicknesses: 1.6 mm, 2.4 mm and 3.2 mm.
Introduction
Quad flat no-lead package is designed so that the thermal pad is exposed on the bottom of the component. This creates a low thermal resistance path between the die and the exterior of the package and provides excellent heat dissipation from the component to PCB. Thermal vias in the PCB thermal pad are typically used to conduct the heat away from the device and to transfer effectively the heat from the top copper layer of the PCB to the inner or bottom copper layer or to the outside environment. A cross-section view of QFN and PCB thermal vias is shown in Figure 1.
Figure 1: Cross-section of QFN and PCB structure.
There are several publications about the PCB layout guidelines for QFN packages requiring thermal vias[1-2]. Some recommend thermal vias in the solder mask defined thermal pad[2] while others place the thermal vias directly on the thermal pad without any solder mask[1]. The solder mask around the via can keep the solder away from the via and prevent it from flowing into the via. However, the solder mask ring tends to create more voids or unsoldered areas at the thermal pad. On the other hand, the solder can flow into the thermal vias if there is no solder mask ring and result in solder loss and solder protrusion onto the secondary side, which can interfere with the assembly process and become a quality issue. In this paper, we will discuss the impact of via design, board design and process parameters on solder protrusion at the thermal pad’s vias. QFN voiding is a known industry challenge with many publications[3–6]. The influence of via design and processes on voiding will also be presented in the paper.
Experimental Details
Test Vehicle and Components
A QFN test vehicle was designed for this study. The test vehicle had the dimension of 177 x 177 mm. The board surface finish was immersion silver (I-Ag). Three different board thicknesses of 1.6 mm (62 mil), 2.4 mm (93 mil) and 3.2 mm (125 mil) were investigated. The image of the test vehicle is shown in Figure 2.
Figure 2: Flex QFN test vehicle Rev 2.
Six different QFN packages with different pin counts and component body size were included in the test vehicle. Both single row and dual row QFN components were studied. The QFN pitch varied from 0.4 mm, 0.5 mm to 0.65 mm. The QFN component body size ranged from 3 x 3 mm to 12 x 12 mm.
Design Variables
Many via variables were designed into the test vehicle, including via size, via pitch and via design. Five different via sizes were investigated. They were 0.20 mm (8 mil), 0.22 mm (9 mil), 0.25 mm (10 mil), 0.30 mm (12 mil), and 0.51 mm (20 mil). Via spacing was 0.5 mm, 1 mm and 1.27 mm. Most through hole vias with no solder mask ring were used while some vias were designed with the solder mask around the via.
Process Variables
Besides the component, board thickness and via design variables, the study also included two different stencil designs. Window pane aperture opening and 1:1 pad aperture opening stencils were used. For the window pane design, the solder paste was printed away from the vias except at the 0.5 mm via pitch locations. For the 1:1 pad design, the paste was printed over the vias. In addition, the boards were reflowed using air and nitrogen, and were reflowed using two different reflow ovens.
To read this entire article, which appeared in the November 2016 issue of SMT Magazine, click here.
Suggested Items
Hentec/RPS Publishes an Essential Guide to Selective Soldering Processing Tech Paper
04/17/2024 | Hentec Industries/RPS AutomationHentec Industries/RPS Automation, a leading manufacturer of selective soldering, lead tinning and solderability test equipment, announces that it has published a technical paper describing the critical process parameters that need to be optimized to ensure optimal results and guarantee the utmost in end-product quality.
Empowering Electronics Assembly: Introducing ALPHA Innolot MXE Alloy
04/16/2024 | MacDermid Alpha Electronics SolutionsIn the rapidly evolving electronics industry, where innovation drives progress, MacDermid Alpha Electronics Solutions is committed to setting a new standard. Today, we are pleased to introduce ALPHA Innolot MXE, a revolutionary alloy meticulously engineered to address the critical needs of enhanced reliability and performance in modern electronic assemblies.
New Book on Low-temperature Soldering Now Available
04/17/2024 | I-Connect007I-Connect007 is pleased to announce that The Printed Circuit Assembler’s Guide to… Low-temperature Soldering, Vol. 2, by subject matter experts at MacDermid Alpha Electronics Solutions, is now available for download.
Inkjet Solder Mask ‘Has Arrived’
04/10/2024 | Pete Starkey, I-Connect007I was delighted to be invited to attend an interactive webinar entitled “Solder Mask Coating Made Easy with Additive Manufacturing,” hosted by SUSS MicroTec Netherlands in Eindhoven. The webinar was introduced and moderated by André Bodegom, managing director at Adeon Technologies, and the speakers were Mariana Van Dam, senior product manager PCB imaging solutions at AGFA in Belgium; Ashley Steers, sales manager at Electra Polymers in the UK; and Dr. Luca Gautero, product manager at SUSS MicroTec Netherlands.
TE Connectivity Orders Hentec Industries/RPS Automation Pulsar Solderability Testing System
04/09/2024 | Hentec Industries/RPS AutomationHentec Industries/RPS Automation, a leading manufacturer of selective soldering, lead tinning and solderability test equipment, is pleased to announce that TE Connectivity has finalized the purchase of a Pulsar solderability testing system.