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Common QFN Issues and How to Avoid Them

At Bittele Electronics, we are fully dedicated to providing straightforward, efficient, low-cost, and customer-focused High Quality PCB Assembly. One of our main goals along this theme is to recognize and rectify any potential faults that could occur at any point in the PCB Assembly Process. When QFN packages are present on a board, we take extra care to ensure the integrity of the project, since QFN Reworking can be rather difficult.

As a PCB designer, your main responsibility is simply to ensure that the footprint in your CAD design matches the land pattern indicated on the QFN part’s datasheet, and that all spacing and tolerance requirements fall within Bittele’s guidelines. For a detailed view of our PCB Fabrication and PCB Assembly capabilities, please see our DFM Guidelines and DFA Guidelines documents.

This article will proceed with a brief overview of the specific areas that have some potential to cause a fault, as identified through more than a decade of industry experience. These are the most common issues that we notice at Bittele through our pre-fabrication DFM Checking process and our post-assembly X-Ray Inspection methods. Clients who choose our Complete PCB Assembly service for their QFN components should familiarize themselves with these common issues, and design around them whenever possible.

Via in Pad

If your PCB design specifies vias within SMT pads, we call the associated production process “Via in Pad” or “Active Pad”. In these cases, there is a risk of solder leakage through the pads during assembly, resulting in a corrupted solder joint that must be reworked after the Reflow Soldering process. In order to combat this difficulty, a non-conductive epoxy is generally used to fill the via hole, while the walls remain plated for electrical conductivity. If the vias are small enough and few enough, it may be possible to just fill the holes with solder mask with no risk to your boards.



Solder Bridging

A short may occur during assembly if there is an insufficient amount of Solder Mask between pins. Designing boards without properly distributing the weight of the components can also contribute to this issue, so be sure to check on the physical aspects of your parts as well as their electrical attributes. It is very important to design the pads making sure to leave enough space between them and the solder mask layer. Component alignment issues may also occur if a proper X-Y (Centroid) File is not implemented, as it is very important to the SMT assembly. For a detailed example on how to generate pick and place files on Eagle, Altium and ORCAD/ALLEGRO, please refer here.

Clearances

Proper PCB Component Spacing is of the utmost importance when dealing with QFN packages. Part-to-part spacing is not usually an issue in our experience, but in an effort to meet increasingly stringent HDI requirements, it is not infrequent for us to receive designs that violate our part-to-hole or part-to-board-edge requirements. If copper traces are pulled too close to the edge of the board, damage is likely to occur when the individual PCBs are routed from the larger Panelized PCB.

Plating voids

Insufficient copper plating on the inside walls of holes on the PCB. The can cause the boards to be defective as the electrical current will not be able to pass between the layers. We can avoid this by ensuring that the holes are thoroughly and properly cleaned after drilling. This helps to prevent any contamination of the material or air bubbles inside of it.

De-Wetting / Non-Wetting

De-wetting can occur when the molten solder does cover a lead or pad, but then retreats and leaves behind a mound of solder. Non-wetting occurs when the solder only partially covers a surface, leaving some exposed copper behind. Both of these issues can be avoided by making sure the component parts are still within their usable shelf lives and the flux being used must not be past it’s prime (after long hours of use).

If you have any further questions about our QFN Assembly capabilities, or to get started on your project with Bittele today, you can Contact Us over email at sales@7pcb.com, or toll-free at 1-888-812-1949. Bittele Electronics is a Canadian PCB Assembly service provider, based out of Toronto, but serving clients worldwide since 2003.



Soft Gold Surface Finish
In addition to our various hard gold surface finish options for PCB Fabrication, Bittele Electronics also offers a soft gold PCB surface finish that is very well suited to wire bonding operations. The soft gold used in this process can easily form strong metallic bonds with standard copper traces and wires. The strong bond of gold and copper allows for more conductive connections when leads are soldered to the board during the PCB Assembly Process.

The process for applying a soft gold surface finish is similar to hard gold; in both cases, an electrolytic process is used to apply the finish, but the soft gold process requires that a Solder Mask first be applied. When soldered, soft gold remains in the alloy, and produces a stronger welded joint at the point of soldering or wire bond.

The Soft Gold Surface Finish makes for a particularly efficient process in Complete PCB Assembly projects where wire bonding is required. With our High Quality PCB Assembly options, you can trust Bittele to handle your project from start to finish.

When you send in your PCB Design Files for a quotation, please be sure to note in a Fabrication Drawing exactly which areas on the bare PCB will require the Soft Gold finish. Please note that the associated cost depends on the specific amount of soft gold used in bonding.

surface finish options

If you have any further questions about our PCB Surface Finish capabilities, or to get started on your project with Bittele today, you can Contact Us over email at sales@7pcb.com, or toll-free at 1-888-812-1949. Bittele Electronics is a Canadian PCB Assembly service provider, based out of Toronto, but serving clients worldwide since 2003.



Hot Air Solder Leveling

Hot Air Solder Leveling (HASL) is one of the most common types of PCB surface finishes used in the industry today. HASL finishes are composed of solder, with proportions of approximately 63% tin and 37% lead, but HASL can also be used for Lead-Free PCBs, with only minor changes to the process. This finish is applied by first dipping the circuit board into a molten pot of the tin/lead alloy after the Solder Mask has been applied. Next, a Hot Air Leveler (HAL) removes the excess solder, using hot air knives to leave behind only the thinnest possible layer. This remaining layer of solder protects the traces underneath from corrosion, while easing the task of soldering components to the board during the PCB Assembly Process by pre-tinning the pads on the board. HASL is a very cost-effective surface finish compared to other types of finishes, and thus is considered a great choice for general purpose boards.

Lead-Free Hot Air Solder Leveling (LF-HASL) is similar to HASL in appearance and usage; however, the solder in this case contains a mix of 99.3% Tin and 0.6% Copper. This alloy results in a higher melting point for lead-free solder, when compared with leaded solder, requiring a slight modification for the Reflow Soldering process in our Complete PCB Assembly solution. LF-HASL is a replacement for leaded solder, used when a lead-free or RoHS Compliant PCB is required. Please note that a High-Temperature Laminate Material is needed for applying this finish; otherwise, the process is identical.

In the past, HASL was one of the most popular surface finish choices due to its qualities as a low cost and robust solution. Recent fundamental changes in the PCB industry, such as new more complex surface mount technology (SMT), have revealed HASL’s shortcomings. HASL is not suitable for use with fine-pitch SMT PCB Assembly due to uneven surfaces being incompatible with fine pitch components. Recently, lead-free LF-HASL became available, but now there are other lead-free options more suitable for a high-reliability product.

Pros
  • Low-Cost
  • Widely Available
  • Easily Re-Worked
  • Excellent Shelf Life
Cons
  • Uneven Surfaces
  • Not Ideal for Fine Pitch Components
  • Thermal Shock
  • Not Ideal for Plated Through-Hole (PTH)
  • Poor Wetting
  • Solder Bridging


Bittele offers a variety of surface finish options, all for a standard price, including HASL. Please note before ordering that we quote lead-free (LF-HASL) by default; there is no difference in price or lead time between the two. In fact, for your convenience, Bittele offers a selection of no-cost-added surface finish options, as a part of our PCB Options at a Standard Price. The chart below shows basic attributes for each finish we have available.

surface finish options

For more information about our surface finish options, or to get started on your PCB project with Bittele today, please feel free to Contact Us any time! We can be reached via email at sales@7pcb.com, or toll-free at 1-888-812-1949.



Common Gerber File Issues, and How to Fix Them
Gerber files are an essential requirement for any PCB Fabrication or PCB Assembly project, including Bittele’s Complete PCB Assembly service. Of all the many types of PCB Design Files, Gerber files are perhaps the most important since they describe the physical attributes of your boards in a way that Bittele’s PCB fabrication machinery can directly process. Even for assembly-only projects, we still very much prefer to have the Gerber files on hand for the DFM Checking that we provide as a part of our comprehensive North American Quality Management standards.

Since PCB design is a complex process with many variables, it is of course not uncommon that some errors occur in a client’s Gerber files. Bittele’s team of PCB Customer Service Experts are extremely adept at catching these mistakes before production begins, but this can sometimes cause a delay in the PCB Assembly Process while clients correct the issues. As such, this article is intended to highlight some of the most common issues that we have noticed over our years of experience in dealing with Gerber files. We hope that this will allow our clients to develop a sort of checklist for their Gerbers, to ensure that they are correct and complete before submitting the project for quotation.

1 – Missing Board Outline

The board outline is perhaps the most simple and straightforward piece of your Gerber files, but it is still necessary to describe the actual bounds of the board edge to our fabrication machines so that the individual boards can be routed from the panel. When you generate your Gerber files, some CAD software packages will not produce a board outline by default, and must be specifically instructed to do so. The board outline can be described by its own Gerber file, or as a part of the general drill file(s), but it must be present somewhere, so make sure to double-check for it to save on communication time.

2 – Missing Excellon / NC Drill File

Similar to the first point, many PCB Design CAD software packages have a separate option for generating drill files and other Gerbers. This can make it easy to miss the drill file, which is required for creating any through-hole footprints that might be present in your design. We sometimes see a drill map submitted in place of the Excellon / NC Drill file, but the drill map contains only supplementary information that cannot be directly translated to our fabrication machines. Make sure to include drill files with the .xln or .drl extension for with your main Gerber file archive.

3 – Empty or Corrupted Gerber Data

Erroneous or empty Gerber data is usually very noticeable from a cursory look at the Gerber files, so it is highly recommended to use a Gerber Viewer to verify that your generated Gerbers match your design before submitting the package to Bittele. These issues normally result from incorrect settings within your CAD software, so check carefully that the Gerber generation is correctly specified. Bittele provides a number of convenient guides for correctly generating Gerber files in Eagle, KiCAD, Altium Designer, and AutoCAD.

4 – Incorrect File Format

The industry standard Gerber format is RS-274X, and this is the format required for Bittele’s PCB Fabrication process; we can also accept ODB++ Design Files for PCB Fabrication. We sometimes see clients who export to older formats, or who send their designs in using the file format of their specific CAD software, such as .brd for Eagle. Before you make your submission, we recommend that you double-check that your Gerbers are in the correct format.

5 – Ambiguous File Names

For Multilayer PCBs and HDI PCBs, the order of the copper layers is often unclear from the file names themselves, so we highly recommend providing a simple PCB Stackup to make this information as clear as possible. The stackup can be included in your Fabrication Drawing, as its own PDF or Excel file, or directly within a mechanical layer of your Gerber files.

6 – Mis-Match with Other Design Files

We understand that the PCB design process occurs in multiple stages, and by the time you are ready to submit your design files to Bittele your design has likely gone through multiple revisions. Unfortunately, this might make it easy to overlook a revision on one of your design files, and the resulting discrepancies can cause confusion and potential delay during production. Before you submit your designs, it is a good idea to double-check that revisions have been reflected on your Gerber files, your Bill of Materials (BOM), your Centroid file, and any Fabrication or Assembly drawings you might choose to provide.

7 – Composite Layers

Some CAD software packages have a tendency to split 1 copper layer into many Gerber files. This phenomenon is most common for layers with filled areas, where the software will generate one file for copper pour, one for traces, and one for clearances. The issue results from a specific option within your CAD layout software, so make sure to double-check your files with a Gerber Viewer before submission, and correct your CAD software settings if necessary.

8 – Vectorized Pads

Vectorized pads occur in some cases for surface mount pads on the solder mask or paste mask layer of your PCB. Rather than generating the pad as one solid object in the associated Gerber file, your CAD software might output a cluster of smaller vectors that together form the pad. This issue can be handled by Bittele’s production team, but requires some specific engineering attention and therefore additional time during manufacturing. To make things more efficient for everyone, we recommend setting your software to generate “Flash Pads” instead of “Vector Pads”.

If you double-check all of the above points before making your design submission to Bittele, you can guarantee the most efficient possible quoting and PCB production process. As previously mentioned, our team will catch the vast majority of Gerber files issues before they can impact the production of your boards. This is one of the main advantages to our meticulous quality standards for PCB Assembly in Canada. Still, the process works best when all parties work together for the best possible build.

If you find yourself with any remaining questions about your Gerber files, or to place an order with Bittele Electronics, you can Contact Us any time! We can be reached via email at sales@7pcb.com, or toll-free at 1-888-812-1949.



Eliminating Ambiguity in Your PCB Designs
At Bittele Electronics, our specialty is efficient and high-quality Complete PCB Assembly, and we do our utmost to work with our clients every step of the way to make this process better. We pride ourselves on our straightforward PCB Assembly Process, where we handle every step of the process, including DFM Checking, Parts Procurement, PCB Fabrication, Electrical Testing, PCB Assembly, and Final Inspection / Testing. Bittele provides high-quality PCB Assembly in Canada, and we have been industry leaders in PCB service since 2003. Through those years of experience, we have tailored our processes to be as simple and efficient as possible for you, our valued client. In turn, there are a few strategies that you can employ as a PCB designer to make the process even more efficient and to ensure a perfect build the first time.

When you choose our Turnkey PCB Assembly service, all you need to do is send along your PCB Design Files, and we send back a tested and fully functional board. As such, our process operates at its maximum efficiency when your design is as clear and unambiguous as possible. Bittele’s Quality Management team is extremely adept at catching these types of errors before production begins, but inconsistent or ambiguous data in design files will prompt additional questions, and potentially increase the overall project lead time. Most commonly, these types of issues arise when information in one document disagrees with information in another, so a thorough check that all of your files reflect the most recent design revision can pre-empt many questions during production.

Aside from general consistency between design files, there are a few more specific strategies for eliminating ambiguity in each of the four main types of design files that Bittele requires:

Bill of Materials (BOM) Clarity

Most CAD programs for PCB Layout offer automatic generation for your Bill of Materials (BOM) based on Reference Designators, footprints, and component values specified during the schematic design phase. These options can be used to save on time and minimize common errors such as mismatches between the quantity and the number of reference designators. That being said, a manual review of the software-generated BOM is still essential to ensure its overall clarity.

Some of the most common BOM issues that Bittele has noticed in the past are listed below:
  • Incomplete, ambiguous, or missing Part Numbers
  • Quantity to reference designator mismatches
  • Part number to description mismatches
At the very least, your BOM should include complete information on Quantity, Reference Designators, and Part Number on each line. To further protect against potential delays or confusion, it is helpful to include information about Manufacturer, Description, and Additional Instructions where applicable.

sample bill of materials (bom)

Centroid Clarity

Clarity in the Centroid File – also known as XY-Coordinate or Pick & Place file – is extremely important, since this data will be directly used to program Bittele’s Pick and Place machines for Automated Part Placement. Centroid data is Generated Directly by the CAD Software used for PCB layout, but this does not make it immune to inaccuracy or ambiguity. The following issues are the most common for Centroid data:
  • Disagreement with the BOM, usually due to project updates not reflected in one document
  • Missing parts, usually due to errors in the CAD software
  • Inaccurate coordinates, usually due to a Gerber file update without generating a new Centroid
A sample Centroid file is shown below:

sample centroid

Gerber File Clarity

The first thing to note about Gerber Files and their role in PCB Assembly is the Silkscreen Layer. This layer shows reference designators for specific components beside their associated Land Patterns. During Bittele’s initial DFM / DFA check sequence, these reference designators are compared with those in the BOM and Centroid files to ensure that any errors in one file are caught early and brought to the client’s attention. The silkscreen layer also contains markings for the Orientation of Polarized Components, which can help to avoid questions and potential issues during assembly.

Some High-Density Interconnect (HDI) boards do not have enough room between components for a silkscreen layer, and some clients simply do not want to include silkscreen markings on the finished board for aesthetic reasons. In this case, Bittele will require assembly drawings or mechanical Gerber layers to act as a substitute for the silkscreen layers.

Issues around Gerber file clarity in PCB Assembly are often a result of design revisions that are not reflected across all files, causing a discrepancy between the Gerbers and the BOM, Centroid, or Assembly Drawing. Notes are sometimes copied from a previous design to save time, but design differences are sometimes overlooked in this case; it is recommended to use an empty notes template and fill it for each project individually to minimize these sorts of errors.

Assembly Drawing Clarity

Assembly drawings are a supplementary file, used to offer additional clarity on the required PCB assembly, and to describe in detail any special requirements for the project in question. Most PCB Layout software suites include an option for generating assembly drawings, which will generally output to PDF both a top-down and a bottom-up view of the board, as below:

sample assembly drawing

A basic assembly drawing, such as the one shown above, can help to clarify specific placements and DNI (Do Not Install) parts, and is sometimes necessary to provide more information about special requirements. Processes such as Functional Testing, as well as specifications for temperature tolerances, flux and solder types, or manual assembly parts, should be included in the assembly drawing. These types of specifications can be added on a separate page of the PDF document or simply included in the margins of the drawings themselves.

Bittele Electronics recommends including at least a basic assembly drawing with any PCB Assembly project submission. As a general rule: the more information that is included on the drawing, the fewer the number of questions asked during assembly.

If you still find yourself with any questions of your own regarding clarity and ambiguity in your PCB designs, please feel free to Contact Us any time! We can be reached via email at sales@7pcb.com, or toll-free at 1-888-812-1949.

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