Do people size their stencil for larger components, then use a needle dispenser for smaller components? It seems more flexible than step stencils. Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis.
He has vast experience in SMT equipment, materials and processes.
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics.
He is a pioneer in the science of reflow.
Transcript
Phil
And welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers pick and place or place and pick, if you like to do things in reverse. And we are here to discuss your process, problem situations and dilemmas. And what's today's input Jim?
Jim
Oh well, Phil, this is a rare treat. We have had a listener who has come back and actually suggested an alternative or further refinement of something that we taught.
If you remember way back when we talked about the problem of printing different sized components such that some big components need a lot of paste and we would like to use a thicker stencil. Micro components need less paste and we like to use a thinner stencil.
What we talked about principally was the ideal solution which is step stencil, but there are limitations. But this listener says, do you know any people who size their stencil for the larger components, then use a needle dispenser for the smaller components. Yes, it might be slower but it seems more flexible than step stencils.
Now, this is unique. We have certainly heard of people using dispensing to augment stencil printing. Probably the first time we saw it was way back in the beginning with pen and paste, or intrusive reflow, or reflow of through hole, where people would add additional paste with a dispenser because they didn't feel that they could print enough paste to get good hole fill on the through hole components.
Phil
I will invoke the wisdom of the revered Dr. Hemsley J. Zapfard who said that the only absolute is that there are no absolutes. So yeah, you could probably do it this way. Have we run into anybody recently doing it... No.
Jim
I have never heard anybody doing this.
Phil
Actually the other way around the inverse,
Jim
Print the entire board with a thinner stencil and that means print for both the large components and the small components with a thinner stencil and then augment the volume of paste for your larger components with the dispenser. What I am assuming in this comment is that they don't have any apertures at all for their smaller components.
They have a thick stencil, they print only the large components and then they dispense the entire volume of paste for the smaller components with a dispenser and no, I haven't heard of anybody doing it.
Phil
It sounds awfully cumbersome adding a whole another process and tools.
Jim
Well, what other choices, let's review. We said step stencil is the best way to do it. It has its definite and limitations, spacing and the maximum amount of step you can put in. The next most common is to stay with a thick stencil but reduce the apertures for your smaller components.
We run into limitations with tall and narrow apertures if they get to the ultra miniature components. Less than 0.5 millimeter area raise and 0201s and 01005s.
Another alternative is to use the thinner stencil but overprint your larger components. I am again still staying with one process one stencil. Therefore you don't have any stencil design, area ratio issues on your stencil. Then we get to the one we just described. Another augmentation process is to use solid solder slugs... preforms.
You use a thin stencil, you print the entire board but you add additional paste volume for the larger components with the solid solder slugs which are on tape and reel and placed by your place machine.
The belt and suspenders is to use two completely different stencils and two printing machines. With the thin one first and then have a second thicker stencil that's etched out on the bottom over the thinner deposits. But since most don't want to invest in two stencil printing machines in one line that's probably the most complex.
Yes I don't see any reason why you couldn't, but dispensing is slowing down the process. Dealing with solder paste in cartridges has its own set of problems, but people do dispense solder paste to augment screen printing and stencil printing.
Although we have never seen this exact application, we have seen it for pin and paste, reflow of through hole and augmentation of the larger components in printing with a thinner stencil.
Phil
Remember, whatever you do, however you are doing it, don't solder like it my brother.
Jim
And don't solder like my brother.
Comments
Square or rectangular preforms, supplied in a tape in reel format has been used successfully to augment solder paste volumes on large area pads, RF shielding or when a square through hole pin is used in a round hole since the late 1990's. This is a tried and true process that requires no additional capital, and does not increase the thru-put cycle time.
Mitch Holtzer, Alpha Assembly Solutions
We do see also, with our customers, a serious trend to combine current screen printing and paste jet dispensing in the same production line. The jet dispensing technology can only replace screen printing on low volume (prototyping, NPI,...). For such low volume, the combination with placement on a single platform can reduce the overall equipment and operations requirements.
The best combination seems to screen print the board with regular stencil for the larger components and then dispense the small volumes (small dots, fine pitch) with the solder jet dispensing.
Such best combination will highly depend on the end user specifics (mix ratio) and each project shall be properly tuned. Solder jetting is not any more requiring very specific process skills and can be run reliably on the shop floor with regular operations and commercially available pastes.
Pierre Marechal, Essemtec USA LLC
Many of our customers optimize their stencil design for smaller devices and use our on-board dispenser to supplement areas where additional solder volume is required. This is an ideal cost effective solution for this application. It also offers the flexibility to use one or two heads, paste and/or glue. The dispenser can also be used for prototyping purposes, aka stencil-less dispensing applications as well. The precision is excellent and the cost is a fraction of alternative automated solutions. The on-board dispenser within the printer platform gives you the best of both worlds, the precision, speed and material flexibility of the traditional stencil printing process and excellent dispensing control using proven Archimedes screw dispensing.
Mark Brawley, Speedprint Technology
A cost-effective way to achieve the same result is to utilize the on-board dispense capability of your screen printer. We have quite a few customers who use our dispenser for this application. Many printer manufacturers offer this capability. In our case, any dispense grade material will work and allow the user to employ two dispense heads; therefore, paste and adhesive can be applied adding even more flexibility within your screen printer footprint.
In this case, the stencil is optimized for the smaller devices, (eliminating the need for steps) and additional material (paste and/or glue) is supplemented where needed using the on-board dispenser. You have no material restrictions and gain the crisp, consistent definition of the traditional stencil printing process combined with the precise dispense control of a precision Archimedes screw at a fraction of the cost.
Mark Brawley, Speedprint Technology
It's great to be see a discussion of these kinds of applications. From my experience of what people are doing in the industry (and I am sure that they are doing a lot of stuff that I would find amazing) there are a number of times when jet printing (or jet dispensing) can be used to augment a previously stenciled board, e.g. add-on in cavities, dedicated jet printing of small dot/pitch deposits on a board with mixed pad sizes, shielding applications with mounted boards, or even repair applications.
There have been questions about mixing pastes, since dispensed paste often has a slightly lower volume fraction of solder than stencil paste, but studies performed by manufacturers of solder paste and independent institutes have shown that mixing of paste on a board, and even the same deposit/pad does not have to affect reliability.
Keep up the good work with the 'Board Talk' series.
Gustaf Martensson, Mycronic / KTH
Agree that step stencils have come a long way but do have their limitations. We in house manufacture high mix, low/mid volume boards that have 1005/0201 & 0.4mm BGA as well as large parts all on double sided SMT boards. Having reached the limitations of step stencils we have moved to Jet dispensing with a Mycronics solution. The ability to alter a dispense profile and add/subtract paste in a few clicks of a mouse has seen our defect rates at reflow drop dramatically. We can alter a dispense profile on the fly and see the results in real time.
While the time to dispense a board may be detracting to some applications, for us the panel dispense time is still under the parts placement time so our net quality improvement is worth it. Additionally we remove the stencil cost/time from our supply chain and increase our small run/prototype throughput to verification and test.
Don Argent, AVI Electronics
I see thousands and thousands of stencils per year and have done for nearly 30 years - and it seems very common practice to print the RF fence along with everything else. Step stencils have come a long way in the last few years with the advent of laser welded step technology. Its way more accurate than dispensing and can be applied to 3D topologies. I agree that there are cases for dispense where you need a huge amount of paste - but they should be pretty rare if you have a good stencil vendor.
Fraser Shaw, Tannlin
Mydata has been offering a jet printing solution for solder paste for a number of years. Important difference in jetting versus dispensing is that jetting is a non contact way to apply solder paste compared to dispensing. This will eliminate a lot of issues you might encounter with a traditional dispensing process. The initial adopters of jet printing replaced their screen printer with a jet printer for flexibility, fast changeovers, no stencils, ... Nowadays we see more people looking into the jet printing technology to use it as a complement to their screen printing process.
Typically they will use a thin stencil for the small volumes and add solder paste for those components that need some extra. Since it is a non contact way it is easy to add solder paste even at already screen printed area's. Sometimes the design is limiting the users to use step stencils and with a jet printing process you have a lot more freedom. In some cases it can be an enabling technology if you need to apply solder in small cavities or at different levels. We see more and more boards with mixed technology that require alternative solutions and I strongly believe that add on printing will become more popular in the future.
Nico Coenen, Mydata, Sweden
I always find your "Board Talk" sessions interesting. As I work for a dispensing company, I can add a few notes that relate to this topic. What we are seeing is that supplementing screen printing with needle dispensing is quite common in the mobile device industry, mainly in the area's where RF shields are attached.
There are several reasons for this. The stencil print simply doesn't deposit enough paste that is required for attaching the RF shields, so needle dispensing is used after stencil print. Another reason is that there are many components under the RF shields that needle fluid dispensing onto them, such as CSP or PoP underfill. If the shield is attached as part of the initial print/reflow cycles, then it really limits access to these parts because the shield is covering them. By needle dispensing the solder paste after the 1st reflow cycle, then the underfill process is much easier because you can access each CSP/PoP with a underfill jetting system.
One drawback to this is that the solder used to attached the shield must be specified to reflow at lower temperatures than all other solder used. One other area where we see solder dispensers used is when you have any sort of 3D geometry on the board, like say if you have a recessed area on the board, or if stacking components.
Again, thanks for continuing the "Board Talk" series.
