Pin in Paste Process Guide

Pin in paste, also called through hole reflow, prints solder paste into the plated holes and onto the pads of a through hole component and then reflows it in the same pass as the surface mount parts. The process removes a second soldering step, and it removes the hand soldering that usually follows it, at the cost of a paste volume that is much larger than a surface mount pad would need and a set of rules that the stencil and the component have to respect.

Why the Process Is Used

A mixed technology board would otherwise need a reflow pass for the surface mount parts and a wave or a selective step for the through hole ones. Pin in paste consolidates both into one thermal cycle, which reduces handling, reduces the number of times the board is heated and removes a whole process from the line.

The saving is largest where a board carries a small number of through hole parts among a large surface mount population, because the second process would otherwise be set up for a handful of joints.

The process also suits a connector that stands on the board and cannot be exposed to a wave, and it suits a design that uses the hole for a thermal path as well as for a connection.

Paste Volume and the Stencil

The volume required is much greater than for a surface mount pad because the paste has to fill the annular gap between the pin and the barrel as well as the fillet on the pad. The stencil aperture is therefore oversized relative to the pad, and in many designs the aperture covers the hole.

Some designs print over the hole so that the paste is pushed into the barrel by the squeegee, while others print a ring around the hole and rely on the pin to push the paste through during insertion. The two approaches need different apertures and different print settings.

Because the aperture is large, the area ratio is comfortable and the paste releases well. The risk moves to the opposite end: too much paste leaves a large deposit that does not fit into the joint and spills onto the mask after reflow.

Component and Pin Geometry

The pin has to be long enough to reach the paste and to pass through the board, and it has to be straight enough not to displace the paste. A pin that is bent pushes paste to one side and produces a joint that is full on one side and empty on the other.

The stand off between the connector body and the board should allow the paste to expand and the flux gases to escape. A connector that sits flush on the board traps gas under the body and produces a large void that can be seen as a blowhole beside the pin.

The connector material has to tolerate the reflow temperature. A plastic body that softens during the profile will move the pins after they have been placed, and the resulting joint is offset even though the placement was correct.

Placement and Insertion

The connector has to be placed with enough force to push the pins through the paste and into the holes without disturbing the deposit. A high speed placement head applies a force that is set by the machine, and the setting has to be matched to the connector.

Where the pins are long and the connector is large, the placement can be done with a press after reflow of the surface mount parts, which requires a second handling step and defeats part of the purpose. The choice between the two depends on the pin length and on the connector design.

Alignment is checked at the pin rather than at the body, because a body that is moulded to its own tolerance can sit correctly while an individual pin is off centre. Vision alignment that looks at the pins is more reliable than alignment to the body outline.

Reflow Profile for a Mixed Assembly

The profile has to satisfy two requirements at once. The surface mount joints need the standard profile, while the through hole joint needs enough time and heat to fill a barrel, which has a much larger thermal mass than a pad.

The usual solution is a longer soak and a slightly longer time above liquidus, so that the barrel reaches temperature. A profile that is at the minimum for the surface mount parts will leave the barrel joints only partly filled.

The volume of paste in a through hole joint can also produce a pressure effect during reflow. The flux gases have to escape through the paste, and a rapid ramp traps them, which is why pin in paste is more sensitive to the ramp rate than a surface mount only board.

Defects and Their Causes

Incomplete fill is the most common defect. It comes from too little paste, a hole that is too large for the pin, a profile that is too short or a pin that has displaced the paste rather than passing through it.

Solder balls and spatter appear around the connector when the paste is heated too quickly or when too much paste has been printed. The excess is thrown out of the joint and lands on the mask, where it forms isolated spheres.

Voids inside the barrel are produced by trapped flux gas. A connector that sits flush, a paste with a high solvent content and a fast ramp all contribute, and the void content is best checked by X ray on a sample rather than inferred from the fillet.

Design Rules for the Layout

Keep the pasted holes away from a large thermal mass where possible, because a hole beside a ground plane takes longer to reach temperature than one in an open area. Where the connector must sit over a plane, the plane should be relieved around the hole.

Leave room for the gas to escape under the connector body. A slot in the connector or a small stand off is often provided by the component supplier, and the layout should not block it with a component placed too close.

Provide a test point on the connector pins where the test strategy requires it, since a joint that is made in the reflow oven is more difficult to inspect than a wave soldered one.

Practical Rules

Size the aperture for the barrel volume rather than for the pad, control the pin straightness and the connector stand off, and lengthen the soak for the through hole joints. Check the fill by X ray on a sample.

Record the aperture, the profile and the inspection results with the build records and the defect history, and review the profile control and the selective soldering alternative when a joint cannot be filled by this method.

FAQ

How much paste does a pin in paste joint need? Enough to fill the annular gap between the pin and the barrel and to form a fillet on the pad. The aperture is usually much larger than the pad, and often covers the hole.

Why does the profile need a longer soak? The barrel has a much larger thermal mass than a surface mount pad. A profile set at the minimum for the surface mount parts leaves the through hole joints only partly filled.

What is the most common defect? Incomplete fill, caused by too little paste, a hole that is too large, a short profile or a bent pin that displaced the paste instead of passing through it.