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Help With Lead to Hole Ratio



Help With Lead to Hole Ratio
What is the proper minimum gap between a round conductor lead and the PCB hole that will allow proper solder fill? Where can I find the recommended guideline?
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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
And welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting, the Assembly Brothers. Today we are coming to you from ITM headquarters, high atop Mount Rialto.

We are here to talk about electronic assembly, materials, equipment, components, practices and procedures, among other things. And among those other things today Jim, what is today's question?

Jim

Well, today I am actually going to combine three questions from three different people that all lead to a fundamental design issue in electronic assembly. Here is the first one that comes from D.P. What is the recommended lead-to-hole ratio?

What is the proper minimum gap between a round conductor lead and the PCB hole that will allow proper solder fill? Our circuit board is 062" thick, double-sided. The next question comes from P.N.

Our bare boards are typically 065" thick. How do we calculate the hole diameter for connector pins if the board thickness increases to .150"? The final one is a process issue.

This comes from C.M. We are seeing insufficient barrel hole with one through hole component during reflow. We suspect the problem could be the hole ratio being too small.

Can you point us to a guideline with the recommended through hole component to hole diameter ratio? What can be do to improve barrel fill? All ties together.

Phil
Yeah, the trifecta. The information that we are going to discuss applies to whether you are doing selective soldering or wave soldering because from a design guideline standpoint they both fin.

I believe, Jim, there is an IPC spec that covers this.

Jim
Actually Phil, my feeling is that this is some of the foundation technology that the IPC developed when they first started putting out specs. The most direct one is IPC 2222, sectional design standard for rigid organic printed circuit boards.

They talk about it in terms of the difference in diameter between the pin and the lead. And they give mins and maxs for different levels of difficulty and so forth. My recommendation is, yes, this is really important.

The ratio, or the difference in the size of pin hole, affects hole fill, which is what we are trying to accomplish to get a good, reliable through hole joint. It is affected by a number of factors and this IPC spec and other specs that will be referenced from this give you a lot of insight into that.

Particularly for thicker boards, from my experience people have actually done some optimization. Obviously as the board gets thicker, the degree of difficulty of getting 75% hole fill, or 50% hole fill, or 100% hole fill becomes more difficult.

I felt it was important that this really is the way to design a board. Start with the size of the pin. The IPC specs point out that there are tolerances in pins and hole diameters and so forth that you should incorporate when you are actually specifying a PC board.

Phil
Very good. You did indeed answer all three questions on one hit. That is pretty commendable, Jim. I've got tell you, I am in awe.

Jim
Well, thank you. There are fundamental questions such as getting good hole fill, through hole joints and design issues. To me this is just a foundation background technology for our industry.

I am sure other people will call in with more related specs that give you more additional information. And there will probably some reference to this one that I quoted, that was IPC 2222.

Phil
Well, they always do send in more comments because here we are trying to explain in five minutes.

Jim
And we thank you. We occasionally don't have absolutely knowledge, so it is nice for other people to comment.

Phil
Yeah, very rarely right? I just want to add we will save this topic for another time.

This of courses applies to all fundamental through hole soldering, as we said through hole can be wave, selective and hand soldering. A future topic, we will discuss some of the parameters for intrusive soldering.

Jim
Or pin in paste, pin in hole. Whatever you choose to call it.

Phil
Reflow, through-hole, and an assortment of other names. But that is a subject for another day.

In the meantime, you have been listening to Board Talk, and Phil and Jim. And as you go forward, whatever you're soldering, whatever you do, please don't solder like my brother.

Jim
And don't solder like my brother.

Comments

For best hole fill beyond the lead/hole upper limit, try IPC class 3 for minimum copper in holes and class 2 for rest. Standard size pth then fill even without components, meaning the lower ratio lead/hole is meaningless.
Sten Bjorsell, Shipco Circuits Ltd
For level A (easy) manufacturability:

The Smallest hole should be >=0.25mm diameter over Largest lead; and Largest hole should be <=0.7mm over Smallest lead.

For level B (medium) the values are >=0.2mm and <=0.7mm.

For level C (hard) the values are >=0.15mm and <=0.6mm

For rectangular or square leads, the "effective diameter" is the diagonal measurement.
William D Burton Jr., CID+, Blue Sky Electronics (retired)
For practical purposes you didn't answer the question. I'm sure you will need a disclaimer but we can't all look up the spec and study it. Please explain a general rule that will help board layout people.
Mark Taylor, Sensor Technologies
You might find this article on achieving excellent vertical hole fill of interest. See link below.

http://www.mp-weknowhow.com/?s=hole+fill
Mark Percival, Murray Percival Co
DfM rules that I used for TH boards were:

1- Hole size is lead size + 10mils (+/- 2mils).
2- Pad size is hole size + 22mils (+/- 3mils).
3- Solder mask is 3 mils larger than the pads on all sides.
4- For pad spacing on radial leaded components use same component pitch.
5- For pad spacing on axial leaded components use maximum body length + 100mils + 3x lead diameter.
Glayson Figueiredo, Philips Medical Systems
Good topic, thank you. If you keep metal surfaces clean during barrel filling a better wetting is expected. Comparing with air, a good N2 inerting system can strongly reduce insufficient barrel filling defects.
Luiz Felipe Rodrigues, Air Liquide

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