|Ask the Experts|
October 31, 2017
Recommended lead to hole ratio
What is the proper minimum gap between a round conductor lead and a PCB hole that will allow for a proper solder fill? The circuit board is .062 inches thick and double-sided.
|Expert Panel Responses|
The lead to hole ratio has to consider the methodology of component insertion and whether or not intrusive reflow or pin in hole technology will be used. Additionally one must also consider whether the process is a lead-free process or a leaded process. All of these must be taken into consideration during the design of the product. Lead-Free solder does not wet the surfaces as easily as the tin/lead solder, therefore more heat, both in solder pot temperature and top side preheat, is necessary to allow the solder to flow up into the plated through hole. The rule of thumb for plated though hole filling is the hole needs to be .008 to .020 larger than the lead. The larger hole size was determined by the accuracy of the placement equipment and the accuracy of the board drilling operation. The combination of the two tolerances typically led to finished component hole sizes to be approximately .038" in diameter. Smaller hole sizes can be used for components which are going to be manually inserted. If the component lead is rectangular then use the diagonal of the lead as the largest dimension, and the hole can be sized accordingly. In all cases the leads must protrude the bottom side of the circuit board to break the surface tension of the solder allowing the solder to flow up into the plated through hole through capillary action and fill the hole to meet the requirements of IPC- A-610 Added note: I would not change the hole to lead size ratio for thicker boards. The basic component plated through hole size was selected for ease of inserting through hole components and the basic hole size was 0.038" finish hole size and on 0.060" board this was approx 2:1 aspect ratio and 3:1 for 0.091" boards with the aspect ratio being hole size versus board thickness. Keeping in mind the accuracy of the insertion equipment and such, these were the hole sizes we used at Digital Equipment during my time there as we wanted the plated through hole to be about .006" to .015" larger than the component lead. This allowed the solder to flow up into the plated through holes, through capillary action and the hydrostatic pressure of the wave solder system. Since then the industry has miniaturized the boards and holes and we now have holes which are drilled with 0.006" drills which is a 10:1 aspect ratio but with no components. The component holes are still about .038" and solder fill is still being met according to the industrial specifications.
Vice President, Technical Director
In a study on "Lead Free Intrusive Reflow," SMT Magazine Nov and Dec 2007, we found that a pin to hole ratio much less than .6 resulted in voiding.
Vice President Technology
For consideration of a good solder joint, you will need to consider the following. Solder Finish,Component Lead Mass, Auto or Manual Insertion, Soldering Method (Wave/Paste-in-hole/Soldering Iron/Preheater), Thermal Vents in Artworkand Solder used. Assuming you areWave solderingandhave a 63/37 Lead and HaSL finish, this would be a robust system. We have seen good solder joint with adiametrical differencegap of .007 - .015 When using RoHS solders, they don't wet and follow heat as well. Therefore we depend on capillary action more, and this requires the gap above to be minimal. We target a .005 Minand .010Diametric gap.
Capital Equipment Operations Manager
Specialty Coating Systems
Refer to the Industry Standard for Printed Board Design (IPC-2221). It specifies through hole component pads, holes, and hole to lead clearances.
Sales Manager (Americas)
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