Through-hole parts are sometimes seen as being the domain of hobbyists, but the commercial reality is quite different. Through-hole parts are placed as connectors, pogo pins, power electronic components, transformers, and much more. When these components are soldered into a PCBA, the through-hole lead must meet a certain lead-to-hole diameter ratio, and a minimum amount of solder infiltration is required for acceptability.
What if you are using long leads or clipped leads? Do the acceptability criteria change and what can be done in the PCB layout to adjust for these factors? This article looks at how to handle these points regarding through-hole components to ensure high-quality assembly with reliable through-hole component mounting.
Through-Holes Require Minimum Solder Fill
To ensure sufficiently strong adherence to plated through-hole walls, through-hole component leads require some minimum amount of solder fill along the length of the lead. This includes situations where a component lead is too short to fit entirely through the PCB, so there will not be a solder fillet present on the back side of the board to anchor the component onto the through-hole barrel.
Here we have two possible groups of requirements for through-hole components: when the through-hole lead length is longer than the board thickness, and the lead length has been clipped to be shorter than the board thickness. Both sets of qualification requirements can be found in the IPC-A-610 and J-STD-001 standards.
When long leads are present and they protrude through both sides of the board, there must be a fillet on both sides of the lead. According to the IPC-A-610 standard, the solder fillet should:
- Wet the entirety of the plated-through hole barrel;
- The solder should also wet the lead on the opposite side of the board;
- The minimum acceptable solder fillet should extend up the component lead a minimum of 1 lead diameter or 2.5 mm, whichever is less;
- If bends are present, the fillet should not extend past the bend of the component lead.
Similar requirements are specified in J-STD-001, although wetting onto the land is specifically called out in this standard.
Solder fillets can be seen on these through-hole components.
Short or Clipped Leads
Products that require a low profile typically use SMD components, but these boards may include through-hole components with clipped leads. Most through-hole components will have leads that are much longer than the standard thickness of a PCB, and the leads will poke far beyond the bottom side of the board if the component sits flush on the top surface layer. To reduce the profile, such as in cases where the board will be placed into a stacked arrangement, the leads will be clipped so that there is minimal protrusion below the PCB.
Sometimes, leads are clipped in order to be smaller than the thickness of the board. In other cases, such as with pin headers, the leads might be selectable such that they are always smaller than the board thickness, so they will not protrude through the bottom side of the board. This also means that the bottom-side pads cannot be wetted with solder under the IPC-A-610 standard.
Are short or clipped lead lengths acceptable under IPC standards? According to J-STD-001 in the IPC standards, lead lengths shorter than the thickness of the assembled PCB are acceptable:
A similar statement can be found in IPC-A-610.
This required amount of solder is sometimes called the plated through-hole barrel fill. For short leads that do not protrude through the bottom of the board, the through-hole fill ratio has a minimum value of 75%.
Controlling Solder Application
In the case where long leads are used, you generally do not need to specify the wetting requirements into a plated through-hole. As long as you reference a qualification standard, your assembler will understand the requirements and can advise on any design changes required to hit these. If an automated solder paste dispenser is used, it can be set to control the amount of solder applied to a component land.
For short leads, you may need to restrict the amount of solder paste deposited onto the component land. This could involve:
- Specifying the reduction in deposit volume in assembly notes
- Reducing the paste mask opening around the component land
The 2nd option will reduce the amount of solder mask that is applied through a stencil, but it will not apply in automated dispensing. Before modifying paste mask openings for short leads, make sure you consult with your assembly house.
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