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1 How To Improve Quality and Reduce Cost Using DFX
How To Improve Quality and Reduce Cost Using DFX
Designing for eXcellence (DFX) is the discipline of defining a printed circuit board assembly process resulting in a more robust and reliable product.
Manufacturing and testing need to be taken into account during the design phase, not after com-pleting the design. Design is the time to optimize your printed circuit board assembly (PCBA) so that it is capable of being produced flawlessly at a reasonable cost.
Following a DFX (Design For Excellence) pro-cess prepares the project for full production and treats the prototype as a validation of manufac-turability and testability.
By initiating the DFX process early in the de-velopment cycle, designers and manufacturers eliminate wasteful design turns through planning for manufacturing. If DFX is incorporated into the design cycle, designs will not need to be rede-signed to fix manufacturability issues.
The DFX process helps manufacturers work through:
• PCB material and fabrication issues.• PCBA manufacturing and testing issues.• Supply chain issues (cost and availability).
Designing with the above list in mind and under-standing the areas where automation plays in the development of your printed circuit board assem-
bly should always be considered, but is often forgotten. The simple procedure of proper fiducial placement and edge clearance vastly improves quality and manufacturability.
BOM Component Selection
A “standard” printed circuit board assembly does not exist. Each PCBA has a unique function for a particular product and must be designed to perform that function. The selection of compo-nents which make up the Bill of Materials (BOM) can make or break the window of opportunity for a product. Planning early in the process for the product’s end use application has a major impact and can minimize problems.
One way to reduce time is generating the sche-matic release of the BOM to the assembly facil-ity for initial scrubbing. This early BOM release
Adopting a DFX (Design for Excellence) process enables printed circuit board designers and manufacturers to uncover gaps in product designs before reaching prototype production. Im-portant questions regarding process, testing and material sourcing can be answered ahead of time to optimize manufacturing speed and quality.
Plan for Production, Not Prototypes
Proper fiducial placement and edge clearance vastly im-proves quality and manufacturability.
Author: Jim Pierce, DFX DesignJanuary 2016
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reveals obsoleted or long lead-time components and allows for either substituting or planning around problematic components.
Another benefit to releasing the BOM early in the process is the manufacturer can plan whether reels, strips or trays are the best choice for their pick and place machines. The planning portion takes into account production quantities and re-quired time-to-manufacture.
During the process of scrubbing the BOM, alter-natives are important and should be called out on the BOM when they can be used. This is very common with passive components, especially when a tolerance of 1% is called out where a 5% or 10% will work just as well, and probably offer better value and availability.
In specific cases, long lead times may be re-quired when unique components or custom components are specifically tailored for your com-pany’s application. These components are often costly and require your approval to be acquired. To minimize additional delays, it is best if all of the component information is given to your manu-facturing partner.
Design To Reduce Process Steps
By reducing process steps, companies save manufacturing time, decreasing failure opportu-nities and costs while increasing reliability. One of the first actions to reduce process steps is to determine the optimal method for assembling your printed circuit board. Solder paste is applied through a stencil to the board, then automated placement machines position components on the PCB.
Component packaging is a critical element of component placement. Tape and reel feeders,
tube feeders, or matrix trays feed components into the automated placement machines. Tape and reel is the preferred packaging method for most components.
Inline forced-convection ovens heat the solder paste and the entire PCB to a specific tempera-ture, which melts the solder paste alloy and forms the surface mount solder joints. After soldering, the oven gradually cools the PCBA. Through-hole components (connectors, sockets, etc.) are hand inserted and soldered with a selective soldering machine..
Cleaning, if required, is used to remove flux con-tamination after reflow and selective soldering. Aqueous (water) is used to clean water soluble flux. Monoethanolamine (MEA) Aqueous Solution is used to clean rosin and no-clean flux (water alone will not remove these flux chemistries).
Following the soldering process, PCBA are visu-ally inspected using the globally accepted IPC-610 workmanship standard. Automated Optical Inspection (AOI) is used to detect manufacturing defects for the following:
1. Solder joint defects2. Component lead defects3. Component presence and position4. Correct component 5. Correct orientation (polarity)
Any defects found during AOI are investigated to determine the root cause. After the problem has been diagnosed, the defects are reworked to bring the PCBA into compliance.
Testing uses flying probe or in-circuit (ICT) JTAG / Boundary Scan testers. ICT and flying probe testing primarily detects manufacturing defects such as solder bridges, continuity, and missing components. Also, lifted pins on IC legs (or open balls on BGAs) can be detected, and shorts be-tween adjacent pins of ICs and connectors. Ultimately, functional tests confirm the PCBA works properly.
Fiducials are used to locate the PCBA during assembly, inspection and test. Without fiducials Machine ready component parts on reels.
3 How To Improve Quality and Reduce Cost Using DFX
it is virtually impossible to manufacture a PCBA, especially if it contains fine pitch devices and BGAs.
Proper component spacing throughout a PCBA ensures ease of component placement and insertion, solder joint inspection, and rework. Through-hole components are usually taller than SMT components, so additional spacing may be needed to improve access to the surface mount-ed components and their solder joints.
This completes a high level overview of the basic steps in the manufacturing process. Here is what to consider in board design to optimize the manu-facturing process:
1. Fiducial targets.2. PCBA edge clearance.3. Land pattern design.4. Component-to-component spacing.5. Test points for probe testing.
Collaboration with your manufacturing partner early in the design process cuts time and cost, and improves quality.
Design For Automated Processes
Printed circuit board assemblies are manufac-tured on machines that use edge conveyors to transport and hold the PCBA. If a 5mm edge clearance is not provided along two parallel edges it will be necessary to add a multi-pack frame or use expensive fixtures to hold the PCB in place during assembly, inspection and testing. The 5mm requirement is an industry standard based on the IPC-SMEMA-9851 Mechanical Equipment Interface Standard. All of today’s SMT equipment suppliers use this standard.
There is remediation if a PCBA design does not allow for the 5mm board edge clearance, but it is not the preferred process and adds costs as well as taking more manufacturing time. Early involve-ment with the manufacturer engineering team will help work through these issues.
Adding rails can resolve the problem of not de-signing-in the 5mm board edge clearance. Rails are added to the PCBA when it is fabricated. Printed circuit board size doesn’t matter as much as the shape of the PCB, which has an impact because of the routing / score lines or mouse bites that will need to be added. These added rails will be removed from the PCB once it has been completely assembled and tested.
Fiducial targets are used by the assembly, in-spection and test machines to locate the PCBA when it is in the machine. All PCBAs should have at least two fiducial targets placed in opposing corners on the longest diagonal of the PCB. Fidu-cials should be designed to the industry standard, SMEMA 3.1.
Maximize Design For Electrical Testability
Finally, determine the electrical testing require-ments up front. How the board is designed will dictate the testability coverage, and what type of testing can be performed. Spacing of compo-nents plays a critical role in testing. Design for testability will reduce cost and improve PCBA quality.
Rails added to solve the lack the 5mm board edge clear-ance.
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Probe testing – flying probe or in-circuit – is used to detect assembly problems (e.g. solder shorts, solder opens, etc.). Flying probe testing is the most cost effective approach for low volume pro-duction. Functional testing is used to confirm the PCBA will power up and function properly.
When planning testing and its impact/cost, visu-alize what the testing should accomplish. Every trace, solder joint, component, and connector pin presents an opportunity for failure. Furthermore, PCB size, routing space, package types, compo-nent types, etc., all contribute to the probability of failure. This is why early involvement and collab-oration with the manufacturing team is so import-ant to the success of the project.
All circuit nodes should be accessible. Depend-ing on complexity, most circuit nodes can be accessed on the solder side of the board. If test-points are to be added, then they should be placed no closer than .075 from any device, with a standard pad size of .035.
With components like BGAs, test access is limit-ed to the opposite side of the board through the via pad. The manageable size for these via pads is .015. The most common design mistake today is covering these pads with soldermask, blocking test probe access and drastically reducing the test coverage.
By bringing together the DFX process and good team collaboration, manufacturers can now build a printed circuit board assembly right the first time.
The flying probe detects assembly problems such as solder shorts or opens.
Jim Pierce, Senior PCB Designer, DFX Design
Jim joined Axiom in 2012 after working for DFW Test where he managed a team of design engineers located in Dallas, Tucson, the Philippines and India. Jim’s team focused on developing ATE boards for the semiconductor test equip-ment industry while also documenting design standards. Jim’s mission for Axiom is to provide Design for Manu-facture (DFM) engineering services for greater customer results. Jim now works for Axiom’s sister company, DFX Design. He brings DFX Design deep expertise garnered during his thirty years of printed circuit board design work.
Axiom Electronics LLC ● 1-800-643-6601www.axiomsmt.com
DFX Design ● 1-503-350-4120www.dfxdesign.com