As far as cost shipping and customer service we like all three. I think Digikey get first shot on based on the website alone. Anyone else see this the same way? Also - why dont the larger, volume parts suppliers have just as good BOM import tools. I dont mean to make light of inside sales people but the response can be from slow to never in time to get a quote in to the end customer. Why cant they develop sites to import for production qtys?
Friday, September 12, 2008
Rating BOM Import Tools / Online electronics
Anyone with decent excel skills can align and convert a BOM in Excel file to the text file required and import the BOM for almost instantaneous price and availability from the major component suppliers. Digikey is by far our favorite parts purchasing website. Not the least expensive, but it is the easiest to use, has great links to datasheets and the costs are somewhat made up by the time the site saves you. This has resulted in the majority of our customers using Digikey at the design stage and placing digikey part numbers right in the Bill of Materials. This also allows the buyer to quote or purchase by importing the BOM right into the website for quick price and availability. The only quirk with the Digikey site is when the BOM has incorrect, or obsolete parts that dont import, its very difficult to find alternates and page back and forth between sourcing pages to your order. Other than that their system is golden! Mouser also has this capability but is our second choice for online purchasing and quotes. We tend to go to DK first, then source whats not available from Mouser. Anything beyond that we try and obtain from Newark.
Tuesday, August 19, 2008
Aetec International Fails to Pay for $6700 Worth of Repair Work
I am not in the habit of talking bad about competitors. I have actually done repair work on many occasions for Aetec International and although I find it surprising that they can't fix their own manufacturing mistakes, I would never use this to make a company look bad. However since they basically contracted us to fix their mistakes, with no intention to pay for the work I find it repulsive that this company can continue to use the system this way to defraud the industry of hundreds of thousands of dollars. And although my loss is one of the lowest in a long long list of deceived suppliers, ex-employees, partners, at $6700, its worth warning my colleagues, suppliers and customers about.
Since closing down its Tempe location and operating a plant in Costa Rica, Aetec has not paid us for work done over 2 months ago. They issued us a contract from the address they no longer occupy, they have ignored emails, phone calls and hung up on us when we requested the new address.
" After learning what they have done in the past to other vendors, and the actions of their employees, I have no doubt that there was never any intention of Dan Stuber to pay for our services rendered in June of 2008".
(Despite the cost reductions of cutting all the Tempe Arizona jobs and manufacturing in Costa Rica) to this day they refuse to pay the local Arizona, US vendors for the materials and repair work?
Through our own channels we have located Aetec International. Here is their local address:
They are now operating as ICM Inc.
Aetec is now operating as the following company.
ICM Inc.
4809 East Thistle Landing Drive, Ste. 100
Phoenix AZ 85044
Aetec International , from what I have learned from other distributors and board vendors, has had a long term reputation for not paying their suppliers. Despite being participant, (and I say participants since upper management consists of 1 individual and his lawyer partner) in this form of legalized theft, I won't mention the name of the 4 or 5 remaining employees (yet). I can see through maricopa.gov webstite that some form of legal action has been taken by 3 other individuals, perhaps past employees. I also saw a judgement against Aetec from Arrow Electronics for over $434,000.00 I have personally tried to reach the owner/president of Aetec for three weeks now, with no response.
Here is a link to some of Aetec's civil case history: (they also have justice court cases)
Aetec Civil Legal History
Here is a partial list of US electronics companies and suppliers that Aetec Inc., (ICM Inc. now) has refused to pay for materials or services which are in collections.
Bisco
Redboard
Carlton Bates
KR Anderson
First Phase Technologies
Arrow
To quote more than one source "be very careful dealing with that company". If you have your own case history regarding Aetec International, please comment....
Since closing down its Tempe location and operating a plant in Costa Rica, Aetec has not paid us for work done over 2 months ago. They issued us a contract from the address they no longer occupy, they have ignored emails, phone calls and hung up on us when we requested the new address.
" After learning what they have done in the past to other vendors, and the actions of their employees, I have no doubt that there was never any intention of Dan Stuber to pay for our services rendered in June of 2008".
(Despite the cost reductions of cutting all the Tempe Arizona jobs and manufacturing in Costa Rica) to this day they refuse to pay the local Arizona, US vendors for the materials and repair work?
Through our own channels we have located Aetec International. Here is their local address:
They are now operating as ICM Inc.
Aetec is now operating as the following company.
ICM Inc.
4809 East Thistle Landing Drive, Ste. 100
Phoenix AZ 85044
Aetec International , from what I have learned from other distributors and board vendors, has had a long term reputation for not paying their suppliers. Despite being participant, (and I say participants since upper management consists of 1 individual and his lawyer partner) in this form of legalized theft, I won't mention the name of the 4 or 5 remaining employees (yet). I can see through maricopa.gov webstite that some form of legal action has been taken by 3 other individuals, perhaps past employees. I also saw a judgement against Aetec from Arrow Electronics for over $434,000.00 I have personally tried to reach the owner/president of Aetec for three weeks now, with no response.
Here is a link to some of Aetec's civil case history: (they also have justice court cases)
Aetec Civil Legal History
Here is a partial list of US electronics companies and suppliers that Aetec Inc., (ICM Inc. now) has refused to pay for materials or services which are in collections.
Bisco
Redboard
Carlton Bates
KR Anderson
First Phase Technologies
Arrow
To quote more than one source "be very careful dealing with that company". If you have your own case history regarding Aetec International, please comment....
Sunday, August 17, 2008
Are We Best of the Best and Frequently Perfect?
Interesting thing about low volume manufacturing is that although the opportunity is there to be perfect, (zero defects on a 5 pc order is better than Six Sigma right?) unfortunately, so is the opportunity for disaster. One pin, on 1 IC, on 1 board out of 5 not soldered and depending on your history with the customer, (and very likely if its your first build) it might be find another vendor time!!
Situations that challenge perfection:
This is not to say that the contract manufacturer will not offer the opportunity for mistake.
Are they the "Best of the Best"? How is this gauged?
Are they "Frequently Perfect"? What challenges does the customer present to keep this from happening?
Do they respond effectively to feedback on defects and do they provide adequate root cause and corrective action?
Although the CM can always provide a corrective action, they are not in a position to change the root cause if its design related. Is the customer, or OEM receptive to DFM issues that are root cause for defects?
Is the CM dedicated to continuous improvement?
I can confidently answer Yes, to all of these questions, but hey, I own the company...What does my customer think?? We'd love to know!
Situations that challenge perfection:
- Incorrect Bill Of Material
- Poor Silkscreen and/or drawing (missing designators, missing polarities)
- DFM issues with layout (solder mask, pin spacing etc..)
- ECN's / modifications
- Compressed delivery schedule
This is not to say that the contract manufacturer will not offer the opportunity for mistake.
- Human error
- Less than optimum reflow profile
- Failure to interpret notes correctly (or ask)
- Pin 1 or Polarity Interpretation (diodes and IC's can have many confusing markings)
Are they the "Best of the Best"? How is this gauged?
Are they "Frequently Perfect"? What challenges does the customer present to keep this from happening?
Do they respond effectively to feedback on defects and do they provide adequate root cause and corrective action?
Although the CM can always provide a corrective action, they are not in a position to change the root cause if its design related. Is the customer, or OEM receptive to DFM issues that are root cause for defects?
Is the CM dedicated to continuous improvement?
I can confidently answer Yes, to all of these questions, but hey, I own the company...What does my customer think?? We'd love to know!
Monday, August 11, 2008
Business Partner or Just Another Supplier?
Are you a supplier or business partner to your customer? Any contract manufacturer fortunate enough to maintain a business relationship with another organization for an extended period of time, lets say 2 or more years, to me is a business partner. Not in the true shareholder sense of the word, but as two or more organizations who join forces to foster growth, revenue and profit. Two years is certainly a long enough time to give as well as take. I can honestly say that some of my customers I work with closely with (daily in most cases) I feel like we are partners and I believe that the feeling is mutual. I'd do whatever I can to see that their projects are handled effectively.
Unfortunately, others you may not work closely although within the same organization quite possibly view you as just another supplier. For some who have no firsthand experience working with you, or perhaps have not had the opportunity to see your company go the extra mile, your services may seem a dime a dozen or worse yet, expendable. I have made the mistake of assuming my customers whole organization should think we're great because of continued success with a particular group or engineering division. I have actually found myself being offended by parts of the company who weren't aware of my track record and question our capabilities. I realize now that I need to re-evaluate how I interact. I need to treat each new division, group and individual as if it were a new organization and we're starting off for the first time in business despite my previous success. Hey, I love what I do, I find it challenging and it keeps me going. I am confident that I have one of the best manufacturing team in the business, frequently perfect and always the best.
Unfortunately, others you may not work closely although within the same organization quite possibly view you as just another supplier. For some who have no firsthand experience working with you, or perhaps have not had the opportunity to see your company go the extra mile, your services may seem a dime a dozen or worse yet, expendable. I have made the mistake of assuming my customers whole organization should think we're great because of continued success with a particular group or engineering division. I have actually found myself being offended by parts of the company who weren't aware of my track record and question our capabilities. I realize now that I need to re-evaluate how I interact. I need to treat each new division, group and individual as if it were a new organization and we're starting off for the first time in business despite my previous success. Hey, I love what I do, I find it challenging and it keeps me going. I am confident that I have one of the best manufacturing team in the business, frequently perfect and always the best.
Friday, August 8, 2008
Low Volume, Root Cause & Corrective Action
Applying volume production control practices to low volume or prototype manufacturing is impractical. I am writing here to familiarize customers with some of the variables in low quantity, high mix, complex PCB assembly.
Here is what I can tell you about a quantity of 5 or less being built by contract: Most organizations are not going to program pick and place equipment for quantity of ten or less, let alone buy all parts on tape & reel for low qty builds. Its not that they wont, its that the customer will not want to pay for it.
Some CM's may tell you they'll automate it (because the perception is that robots are more accurate than humans...right?) NO - here's the reality: Every new program set up will require that feeders and feeder pick positions need to be adjusted. The program essentially goes through a "debug" process while the feeder pick is fine tuned. What this means is that parts are not being picked (which means they are not being placed). Parts are possibly being dropped in transition from the feeder to the placement location. The first 3 to 5 boards could very likely have parts missing, parts placed in the wrong direction or orientation or any number of placement errors. Not a big deal getting through the learning curve if you're running 200 boards, but certainly a futile effort if all your building is 3 to 5 units!
We can justifiably reach the conclusion that 3-5 pieces is a highly manual operation whether you waste your time setting up pick and place equipment, or you prepare placement charts for print, place and oven reflow. Ultimately any workmanship errors fall on the contract manufacturer. Root cause, on the otherhand.... Not necessarily.
Human Error
The operator, being human, made a mistake, an oversight. Corrective Action? If you have an experienced hard working assembler or inspector, perhaps making less than 1 mistake every 10000 components placed or checked, is there a root cause or corrective action? If your placing over 600 components of 200 different types, manually, the opportunity for humanity to come through is great....and sometimes does.
Designed For Manufacturing (or Not)
I had one manual placement job in which a BGA device was placed on one side of the board and a device of weight and mass designed to be reflowed on second pass (topside of the PCB) was placed on the opposite side. These parts were designed to be reflowed on the same side, but you have to solder the second on the opposite side by hand because they drop during second pass reflow upside down. The surface mount pins of this device were almost inaccessible when placed next to each other. The operator could not make adequate contact with the soldered lead and the pad. He or she did their best to solder it. It appeared to be soldered but was not. Product ships then test engineering finds the lead to be open..
The root cause is the design flaw, not the operator. The corrective action is to layout both parts on the same side of the board so they can reflow in an oven as they were designed. A corrective action may also be to layout the parts to space them further apart to improve soldering iron access.
My point here in this article is that when you have a highly automated production job, the opportunity to identify root cause, implement corrective action and process improvement is significant. When you are building 3, 5 or even 10pcs of a PCB assembly, the process is highly manual, there often is no "process variable" to improve on. I also wanted to point out that often the root cause and corrective action can be outside of the contract manufacturers control.
Here is what I can tell you about a quantity of 5 or less being built by contract: Most organizations are not going to program pick and place equipment for quantity of ten or less, let alone buy all parts on tape & reel for low qty builds. Its not that they wont, its that the customer will not want to pay for it.
Some CM's may tell you they'll automate it (because the perception is that robots are more accurate than humans...right?) NO - here's the reality: Every new program set up will require that feeders and feeder pick positions need to be adjusted. The program essentially goes through a "debug" process while the feeder pick is fine tuned. What this means is that parts are not being picked (which means they are not being placed). Parts are possibly being dropped in transition from the feeder to the placement location. The first 3 to 5 boards could very likely have parts missing, parts placed in the wrong direction or orientation or any number of placement errors. Not a big deal getting through the learning curve if you're running 200 boards, but certainly a futile effort if all your building is 3 to 5 units!
We can justifiably reach the conclusion that 3-5 pieces is a highly manual operation whether you waste your time setting up pick and place equipment, or you prepare placement charts for print, place and oven reflow. Ultimately any workmanship errors fall on the contract manufacturer. Root cause, on the otherhand.... Not necessarily.
Human Error
The operator, being human, made a mistake, an oversight. Corrective Action? If you have an experienced hard working assembler or inspector, perhaps making less than 1 mistake every 10000 components placed or checked, is there a root cause or corrective action? If your placing over 600 components of 200 different types, manually, the opportunity for humanity to come through is great....and sometimes does.
Designed For Manufacturing (or Not)
I had one manual placement job in which a BGA device was placed on one side of the board and a device of weight and mass designed to be reflowed on second pass (topside of the PCB) was placed on the opposite side. These parts were designed to be reflowed on the same side, but you have to solder the second on the opposite side by hand because they drop during second pass reflow upside down. The surface mount pins of this device were almost inaccessible when placed next to each other. The operator could not make adequate contact with the soldered lead and the pad. He or she did their best to solder it. It appeared to be soldered but was not. Product ships then test engineering finds the lead to be open..
The root cause is the design flaw, not the operator. The corrective action is to layout both parts on the same side of the board so they can reflow in an oven as they were designed. A corrective action may also be to layout the parts to space them further apart to improve soldering iron access.
My point here in this article is that when you have a highly automated production job, the opportunity to identify root cause, implement corrective action and process improvement is significant. When you are building 3, 5 or even 10pcs of a PCB assembly, the process is highly manual, there often is no "process variable" to improve on. I also wanted to point out that often the root cause and corrective action can be outside of the contract manufacturers control.
Friday, June 27, 2008
Apple Ibook Logic Board Video Problem
I had a potential customer ask me about the reliability of our repair process. Here is my email response which I thought had some good info:
First - Yes we do still offer the repair, and I apologize for the lack of response to your calls. I am looking at better, more responsive ways to handle this work and correspond. Email is the best way to reach me in particular. Our repairs are handled by my technician, Sebastian Sarmiento. (his email sebastian@firstphasetech.com ) You can also call him 480-967-1100.
Regarding the success rate and reliability - My theory is that almost all the G3 and G4 computers with this problem have solder paste deposition during manufacturing at the root of the problem. When solder paste is squeegee'd through a stencil on to a circuit board, the consistency in paste volume deposited on each of the BGA (video chips) pads, varies. It varies for a number of reasons, squeegee pressure, changes in paste volume on the squeegee blade. If the chip pads have different volumes of paste deposited, this will mean it has different sized solder balls after reflow.
Now... over time when that chip (probably the hottest device in operation) expands and contracts in the Z direction, over time the smaller ball fractures. So, the best thermo-mechanical solution, although the success rate is harder to determine, would be to remove, reball and replace the device. However, this means that the chip will see 5 thermal cycles of approximately 200 degrees C. Some of the ATI video devices are multichip modules, they are essentially a BGA with 2 memory BGA's on top of them. They will not survive 5 thermal reflow cycles in most cases without reflowing (disturbing) the smaller chips on top.
The ATI video chips that are all 1 package have a much higher success rate. Most times we just reflow the device. The success of this reflow depends on the degree of inconsistent paste was deposited during manufacturing. I have some boards that are still in operation now for over 2 years. On some, very few occasions, I have had units die again in a week or two (although I have not heard of this in well over 4 mos).
Anyway - I can share more by phone but have to run. Our site right now has a Google checkout button on the ibook page, or you can use ebay, or just send in the board. Please send in the downloadable form though so we can identofy your board when it arrives. I can get your board out of here the same day it arrives right now. Here is our online info on the repair:
http://www.firstphasetech.com/ibook-repair-g3-g4.html
Thanks
Tom
First - Yes we do still offer the repair, and I apologize for the lack of response to your calls. I am looking at better, more responsive ways to handle this work and correspond. Email is the best way to reach me in particular. Our repairs are handled by my technician, Sebastian Sarmiento. (his email sebastian@firstphasetech.com ) You can also call him 480-967-1100.
Regarding the success rate and reliability - My theory is that almost all the G3 and G4 computers with this problem have solder paste deposition during manufacturing at the root of the problem. When solder paste is squeegee'd through a stencil on to a circuit board, the consistency in paste volume deposited on each of the BGA (video chips) pads, varies. It varies for a number of reasons, squeegee pressure, changes in paste volume on the squeegee blade. If the chip pads have different volumes of paste deposited, this will mean it has different sized solder balls after reflow.
Now... over time when that chip (probably the hottest device in operation) expands and contracts in the Z direction, over time the smaller ball fractures. So, the best thermo-mechanical solution, although the success rate is harder to determine, would be to remove, reball and replace the device. However, this means that the chip will see 5 thermal cycles of approximately 200 degrees C. Some of the ATI video devices are multichip modules, they are essentially a BGA with 2 memory BGA's on top of them. They will not survive 5 thermal reflow cycles in most cases without reflowing (disturbing) the smaller chips on top.
The ATI video chips that are all 1 package have a much higher success rate. Most times we just reflow the device. The success of this reflow depends on the degree of inconsistent paste was deposited during manufacturing. I have some boards that are still in operation now for over 2 years. On some, very few occasions, I have had units die again in a week or two (although I have not heard of this in well over 4 mos).
Anyway - I can share more by phone but have to run. Our site right now has a Google checkout button on the ibook page, or you can use ebay, or just send in the board. Please send in the downloadable form though so we can identofy your board when it arrives. I can get your board out of here the same day it arrives right now. Here is our online info on the repair:
http://www.firstphasetech.com/ibook-repair-g3-g4.html
Thanks
Tom
Saturday, June 21, 2008
Design for QFN Devices
August 03, 2007
QFN Devices
QFN Layout GuidelinesWhile QFN (quad flatpack, no leads) and DFN (dual flatpack, no leads) packaged parts are becoming more and more common in new component releases, they aren t getting much easier to use. The advantages of the form factor are pretty clear. It allows smaller geometries, better grounding and improved thermal properties over other types of surface mount packages.
Most QFNs have a center metal pad on the underside of the part, typically for grounding or heatconduction. It s this center metal pad that makes this form factor so difficult to use. DFN packages are identical, for the purposes of assembly, except that they have pin rows on only two sides of the part.
QFN Float- The middle of the part has a metal contact pad like most QFN packaged parts. Itmay be there for grounding or heat conduction, depending on the specific part. Thefloat that I'm talking about happens when we lay too much solder paste on the PCBfor that center pad. To a small extent, the height of the solder paste deposit is proportional to theaperture in the solder stencil opening (bigger opening = taller deposit). With mostparts, that isn't a problem because either all of the pads are big enough so that thatratio doesn't have a first order impact, or because all of the pads are the same sizeand will be equally impacted. Since the QFN center pad is a much larger opening in the stencil than the signal pad openings, and the signal pad openings are in the 10 - 20 mil or less range, this deposit height to width ratio will have a firstorder impact. When the opening for the center pad on the QFN is too large, the paste deposit in the center will be taller than the deposits on the small signal pin pads. The part high-centers and never gets the opportunity to contact the signal pads. In some cases, the part will tilt a little sideways and contact some of the signal pads but not all. Solder Paste Stencil Typically, the signal pads should have a standoff height of 2 - 3 mils after assembly. If too much solder is deposited in the center, the part can very easily float up beyond that height and prevent the signal contacts from connecting. To help prevent this, the solder stencil opening should be broken into a series of smaller openings and should cover between 50 and 75% of the pad area. This means that when you lay out your PCB, you need to look carefully at the solder paste layer for your QFN components. If the solder paste layer in the CAD package part library just follows the copper pad pattern or the solder mask opening, you may need to customize the CAD package part library to avoid leading yourself into trouble. To better illustrate the proper way to make your solder paste stencil for QFN parts, I went to our back room and took a couple of photos of good and bad solder paste stencil practices. This is what a just about worst-case stencil would look like. Actual size for this part is 7 x 7 mm. Note how much surface area that the center pad has compared to the row of side cutouts. With most SMT components, it is standard procedure to reduce the size of the paste cutout area in the stencil. In a case like this, it is difficult to reduce it enough and still get even paste distribution. The proper option is to segment to solder stencil area. If you just reduce the paste opening aperture, providing one smaller opening, but don t segment it, you may end up with a part that is still too high in the middle to assure good contact on the signal pads and is also unstable and will likely tilt to one side. With leaded solder, a single 50% sized opening may work because of the wicking properties of lead-based solder. Since lead-free solder does not wick as well, it is very unlikely to work in a RoHS process. In both cases, the most consistently reliable method is to segment the stencil pattern. This is an example of recommended practice. The basic idea is that you distribute a lower quantity of solder over a broader area. You reduce your chances of highcenteringand other problems associated with large paste areas, such as out gassing and spattering. This will give good solder distribution with little chance of high-centering or outgassing problems. Specialized Copper Pad Some parts, especially high-frequency parts, require a segmented copper pad under the QFN. If this is the case, it is important to segment the solder paste stencil to match thecustom pad. It is fairly common practice to use a standard full-size square opening and hope that surface tension will end up distributing the solder in the right places. While that may happen, the chances of it not happening are equally great or greater. For best reliability and buildability, make sure that the openings match your copper layer underneath the stencil openings. Be sure that your stencil openings only fall above the copper and not over any solder-mask or bare-board sections.Larger opposites With larger QFN parts, the opposite problem can occur in the center padarea. When the square opening for the solder paste stencil is fully open on a larger part say 10 x 10 mm or larger the paste squeegee may deform and actually scoop too much of the paste out of the opening. This can lead to uneven paste and solder voids. Both are potential reliability problems. The solution is the same. Segment the stencil opening to create an even paste distribution. SummaryThe QFN form factor delivers a number of advantages over other SMT package form factors. It is generally a smaller part and, with the center pad, can have better grounding and thermal properties. These advantages are partially offset by layout and assembly difficulties. But by following a few simple guidelines, you can use the parts with good confidence. Check the layout guidelines in the component applications notes. Segment your solder stencil opening for the center pad. Make a custom component library for your CAD package if you need to. Then Design away.
QFN Devices
QFN Layout GuidelinesWhile QFN (quad flatpack, no leads) and DFN (dual flatpack, no leads) packaged parts are becoming more and more common in new component releases, they aren t getting much easier to use. The advantages of the form factor are pretty clear. It allows smaller geometries, better grounding and improved thermal properties over other types of surface mount packages.
Most QFNs have a center metal pad on the underside of the part, typically for grounding or heatconduction. It s this center metal pad that makes this form factor so difficult to use. DFN packages are identical, for the purposes of assembly, except that they have pin rows on only two sides of the part.
QFN Float- The middle of the part has a metal contact pad like most QFN packaged parts. Itmay be there for grounding or heat conduction, depending on the specific part. Thefloat that I'm talking about happens when we lay too much solder paste on the PCBfor that center pad. To a small extent, the height of the solder paste deposit is proportional to theaperture in the solder stencil opening (bigger opening = taller deposit). With mostparts, that isn't a problem because either all of the pads are big enough so that thatratio doesn't have a first order impact, or because all of the pads are the same sizeand will be equally impacted. Since the QFN center pad is a much larger opening in the stencil than the signal pad openings, and the signal pad openings are in the 10 - 20 mil or less range, this deposit height to width ratio will have a firstorder impact. When the opening for the center pad on the QFN is too large, the paste deposit in the center will be taller than the deposits on the small signal pin pads. The part high-centers and never gets the opportunity to contact the signal pads. In some cases, the part will tilt a little sideways and contact some of the signal pads but not all. Solder Paste Stencil Typically, the signal pads should have a standoff height of 2 - 3 mils after assembly. If too much solder is deposited in the center, the part can very easily float up beyond that height and prevent the signal contacts from connecting. To help prevent this, the solder stencil opening should be broken into a series of smaller openings and should cover between 50 and 75% of the pad area. This means that when you lay out your PCB, you need to look carefully at the solder paste layer for your QFN components. If the solder paste layer in the CAD package part library just follows the copper pad pattern or the solder mask opening, you may need to customize the CAD package part library to avoid leading yourself into trouble. To better illustrate the proper way to make your solder paste stencil for QFN parts, I went to our back room and took a couple of photos of good and bad solder paste stencil practices. This is what a just about worst-case stencil would look like. Actual size for this part is 7 x 7 mm. Note how much surface area that the center pad has compared to the row of side cutouts. With most SMT components, it is standard procedure to reduce the size of the paste cutout area in the stencil. In a case like this, it is difficult to reduce it enough and still get even paste distribution. The proper option is to segment to solder stencil area. If you just reduce the paste opening aperture, providing one smaller opening, but don t segment it, you may end up with a part that is still too high in the middle to assure good contact on the signal pads and is also unstable and will likely tilt to one side. With leaded solder, a single 50% sized opening may work because of the wicking properties of lead-based solder. Since lead-free solder does not wick as well, it is very unlikely to work in a RoHS process. In both cases, the most consistently reliable method is to segment the stencil pattern. This is an example of recommended practice. The basic idea is that you distribute a lower quantity of solder over a broader area. You reduce your chances of highcenteringand other problems associated with large paste areas, such as out gassing and spattering. This will give good solder distribution with little chance of high-centering or outgassing problems. Specialized Copper Pad Some parts, especially high-frequency parts, require a segmented copper pad under the QFN. If this is the case, it is important to segment the solder paste stencil to match thecustom pad. It is fairly common practice to use a standard full-size square opening and hope that surface tension will end up distributing the solder in the right places. While that may happen, the chances of it not happening are equally great or greater. For best reliability and buildability, make sure that the openings match your copper layer underneath the stencil openings. Be sure that your stencil openings only fall above the copper and not over any solder-mask or bare-board sections.Larger opposites With larger QFN parts, the opposite problem can occur in the center padarea. When the square opening for the solder paste stencil is fully open on a larger part say 10 x 10 mm or larger the paste squeegee may deform and actually scoop too much of the paste out of the opening. This can lead to uneven paste and solder voids. Both are potential reliability problems. The solution is the same. Segment the stencil opening to create an even paste distribution. SummaryThe QFN form factor delivers a number of advantages over other SMT package form factors. It is generally a smaller part and, with the center pad, can have better grounding and thermal properties. These advantages are partially offset by layout and assembly difficulties. But by following a few simple guidelines, you can use the parts with good confidence. Check the layout guidelines in the component applications notes. Segment your solder stencil opening for the center pad. Make a custom component library for your CAD package if you need to. Then Design away.
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