Ask the Experts
May 4, 2018
Selective Printing for BGA Components
We use common BGA packages on many of our low-end products. To save cost, could we print paste only on the pads that have functional requirements?
For example, could we print all the perimeter pads on a 120 pin BGA, but skip printing on 50 or more of the interior pads since they serve no function?
Expert Panel Responses
At what cost to theintegrity , structural, of the component. Could you do it... sure. Should youdo it ... no.
Lets think of this as abldg. I've got 12 vertical columns that support the structure. In actualityonly 6 support the weight of the bldg. So I will leave 6 inches below thoseremaining six columns to save the cost of rebar & concrete. Bldg issupported but not to its maximum capability.
I would say that notsoldering all the contacts, functional or not could present several problems.
You are now relying onthe perimeter pads to support the entire BGA. The lack of physical contact tothe board by the un soldered 50 pads would, I think, create the enhancedpossibility of cracking on the soldered joints. Flex of the component in allprobability would be greatly increased by the lack of the stability of thesolder connection.
When considering theminute cost of using solder paste to secure the component properly by solderingall the contacts you are stepping over the dollars to save the pennies ...
Weighing the costdifferential of the solder versus product failure & customer bad will... Igo with soldering...
Oh, major point wouldbe.... are you a CM? If so is your customer going to allow this.
One mans opinion!
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
In my opinion it is fine if some of the non functionalpoints are not soldered. It should be understood that solder joints apartfrom electrical continuity provides mechanical strength. The usual thumb rulefor PTH is 7 gms per solder joint is minimum required and similar rule can beextended to BGA also. Secondly in case of BGAs it should be ensured that allfour sides have sufficient soldered joints to support the device.
Astra Microwave Products, Hyderabad, AP India
Holds Degree in Engineering, started off as Scientist/Engineer in ISRO (Indian Space Research Organization) in Quality Assurance of Space hardware Electronics Production. Worked in the area of Parts, Material and Process; DPA, FA and Process Qualification for space and ground hardware. Later moved into Private sector and worked in the area of Quality Management Systems & ISO 9001 certification. Currently hold a position as Head-Quality in RF/Microwave Product manufacturing for Defense and Aerospace segment.
We should stick to rule to maintain equal or betterquality, reliability and keep working to reduce cost.The uneven space betweennon-print BGA balls and paste deposited balls will create reliability risk andis not recommended for partial print at all. Even unused pads should be presentin PCB rather eliminating or masking and should get soldered. Standardization,alternate BGA can be worked out for cost perspective.
Supplier Quality Leader
Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.
Ifthe BGA is depopulated but PWB lands are present, sure, you can simply notprint the lands where no balls are present. If, however, a BGA ball is presentat a given location, the land must be printed. The reasons for this arethree-fold:
All this begs the question, just how much would we expect to save? Basedon not printing 50 lands per BGA, the savings would be in the range of 0.1cents per BGA. It's doubtful that the savings from this would be significantenough to offset the cost of a new stencil and the associated costs ofcustomizing the BGA CAD decals.
- First, even if there is no electrical function, the componentmay well rely on the thermal path, and if it is not soldered to the land, therewill be very little thermal conductivity.
- Second, there will be a big impact to assembly success andreliability. If there is no flux on a given ball, it cannot wet the land and collapse.The presence of balls like this will hold the component up higher, possiblyunevenly, which can create deformed joints or even opens in other areas.
- Third, if you do manage to solder successfully, the reliabilitywill be affected.
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Interesting question. I don't see wherethis would be an issue. I would check whether or not an underfill would benecessary to enhance the strength of the attachment, but other than that, I seeno reason why it would be an issue.
Vice President, Technical Director
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
At first sight this lookslike a good approach - however I do not see the savings being significant. Thisis not an electrical connection question but a mechanical approach of theprocess. BGA balls go through a reflow process so you will expect a consistentcollapse at the end of this process. Missing lands and also paste deposits cancause that collapse not to happen in which case you are going to experiencemechanical stress of the component.
You can try to experiment and perform somecross sections to have a better understanding of the effects taking inconsideration, of course the final application of your product is important too(excessive heat, vibration and other mechanical challenges can accelerate apotential failure in the field). Again, I really do not see savings to justifythis approach.
Engineering and Operations Management
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at firstname.lastname@example.org.
Not printing pads to save money? Has somebody tried to calculate how much savings would result? I would love to see that calculation! Unless you save your paste after printing (which you should never do) the miniscule volume not used is what? equal to a fraction of a gram?
The balls are there for a reason, mechanical, thermal or whatever. Your not going to save anything by not printing them. The package chosen is intentional. Nobody is going to waste packaging cost for their product on a center array that is not needed.
Alan Woodford, OnCore Manufacturing Services, USA
What is one achieving by not dispensing paste in certain locations? Are we saving paste / cost - how much? One of the important feature of a BGA is it's ability to self align and depriving paste off certain locations are we compromising on this important feature?
Mahesh V Draksharapu, Aristos EMS Pvt. Ltd, India
Structural integrity of the device is based on the solder joint and it is equally important to ensure that the device will not fail structurally over a period of time. Surely, design & packaging engineers have thought about this while designing the structure of the pads.
More over, heat transfer of a device is another important aspect and in the cases of BGA's it will be transmitted through the solder joints (other than heat sink mounted). Why restrict the transfer through adequate points and induce other thermal related issues on the device?
Is this a serious question? If you want to save money, use tacky flux on the BGA. No solder paste used. I say that with a smile! Seriously, if you only use some of the functionality of the BGA, look for a less powerful one. You could save more pennies with a cheaper component!!
Jerry Wiatrowski, General Dynamics