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Problems With Starved "J" Lead Joints



Problems With Starved
We have insufficient solder joint heel fillets with a PLCC J lead component. We cannot increase the stencil thickness. What do you advise?
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
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
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
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, Pick and Place coming to you from Board Talk headquarters high above the New Hampshire skyline. What kind of question do you have there today, Jim?

Jim
This question comes from M.M. "We have insufficient solder joint heel fillets with a PLCC J lead component. The aperture on the solder paste stencil is one to one or 30 by 100 mils." Pretty big aperture. "Visually, the solder fillet appears okay, but testing indicates that we need additional solder at the heel joint. We cannot increase the stencil thickness due to the presence of TSOPs and QFP packages. What do you advise?"

Phil
We don't know what the stencil thickness is, but apparently it's a little on the shy side for those poor little hungry little J leads.  

Jim
The world has come to fine pitch components dominating everything because of hand-held devices.

You know, even in cell phones they still have the same conflict because they have all those fine pitch components and then they solder an RF shield down which needs huge volumes of paste. So what we have here ladies and gentlemen is a classic case of different solder volume requirements sometimes called mixed mode printing, broadband printing and so forth. And the answer is a step stencil.

Phil
Yes, everything old is new again.  

Jim
The most robust solution, is a step stencil. Where the questioner here indicates, "I'd like to make the stencil thicker, but I have fine pitch component so I can't." So you step up the stencil at your J lead locations, or you step down the other ones using a thicker stencil, whichever is more economical based upon the distribution and the number of the different styles of components.

Phil
And we're assuming you have ample clearance between where you have to step down to accommodate step down. If not, another alternative would be double printing.  

Jim
Yes, double printing, where you print a thin stencil first, then print with a thick stencil which is etched out, relieved for the other ones, but you really need about three mils of difference in order to do that.

Phil
And another printer, too.  

Jim
The next option would be staying with a thinner stencil and over printing.

And people go, "Oh, but won't I get solder balls and stuff off the pad?" and of course that's a risk, but I'd like to point out that overprinting beyond the pads has been a very common practice in intrusive soldering or pin and paste for many years and it can be done with a minimal risk of solder balls and bridges. Particularly, remember, you're dealing with a 50 ml pitch part so the chances of bridging are pretty small.

And of course we have to add the option of using a preform, adding a preform in your placement system or dispensing additional solder paste. You print your paste and then during your placement operation, one of your feeders has a reel of these solder preforms and your placement machines simply picks it up and places it in the wet paste out beyond the edge of your J lead heel where you want that extra solder and then when you reflow, it melts and combines with the solder paste and gives you the extra solder you need.  

Phil
So show some compassion and feed those J leads and feed them often. Beyond that, this is Jim Hall and Phil Zarrow, the Assembly Brothers saying that whatever you're trying to re-flow soldering, whether j leads, gull wings, whatever ...

Jim
Don't solder like my brother.

Phil
Don't solder like my brother.


Comments

If the quantity of J-lead components per CCA and the total quantity of affected components is not too high, you may want to consider pre-tinning the J-Lead components. This can add quite a bit of extra solder to the component leads without significant risk, as well as improve the overall wetting.
Richard Stadem, Kongsberg Defence AS
Use below options to increase solder:
a. step stencil
b. solder preform
c. solder paste dispenser for selective location
d. stencil aperture can increase longer side up to .5mm on masking.

MD Imtiaz, Napino Auto & Electronics Ltd.
Solder Jet printing is a high speed method of depositing solder paste. Jet printers can either deposit paste on screen printed pads, or jet print the entire component and adjust the paste volume to accommodate J leads. A Solder Jet printer can also be used to correct problems from the screen printer. its a win-win.
Christian Vega, Mycronic
If you can't step the stencil then try using solder preforms to add the additional solder you need!
Mark Maheux, Honeywell Fire Safety
Step stencils have come a long way over the past few years. Another option my be the use of preforms depending on the exact location. Some of the current SPI machines offer dispensing paste prior to placement.
Ray Whittier, BAE Systems
Also consider modifying the pad geometry, if that's a possibility. Extending the length of the pad (and hence the aperture) from the heel would deposit more solder where you need it.
Joseph Jorn, Grayhill, Inc.
Try to make the stencil aperture longer (about 0.3 or 0.4mm) for these pins, the solder paste will be out of the pad in this process but during reflow all the excess of solder will go to the fillet. Be sure only makes it longer for the external face not under the component. It will work.

Justiniano Castro, Nagares S.A
Usually I go with overprinting as a first option when working on similar defects, solder paste would flow back towards the terminal and form a good solder joint. Solder balls is not usually a concern when overprinting is not severe.
Labib Saleh, FCT Assembly
"...Then they solder an RF shield down which needs huge volumes of paste". Harwin can help with that bit - we do Shield cans that are not soldered to the PCB, instead you use a series of Shield Can Clips. These hold the RFI shield in place, and the shield can also be removed just as easily should rework underneath be required. The Clips are just SMT soldered to the PCB with your other components. https://www.harwin.com/connector-products/featured/ez-shield-clips/
Wendy Preston, Harwin plc

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