What Process Impact of Substituting SMDs with Paste-In-Hole components?

SMD PIHSomeday, there is one question came from my manager, “Is there any difference or influence on the process and design if we changet the SMDs (Surface Mount Devices) to THDs (Through Hole Devices) type and go through the PIH(Paste-In-Hole) process?”

Later, I realized the question aimed to avoid damage to the connector components caused by customers’ misuse of brute force.

Sometimes, Paste-In-Hole (PIH) is also called Pin-In-Paste (PIP).

First, since the traditional insertion parts through PIH process is necessary to be manufactured with high-temperature reflow process, the part design should fulfill the considerations listed as follows:

  • The PIH parts had better be packaged in tape & reel for mounting by SMT machine. At least, the hard tray should be adopted.
  • The material of PIH parts should be processed under high temperature of SMT reflow process. Generally, PIH parts are required to place on the second side. If the parts go reflow only once, in view of the current lead-free reflow process, the parts should be able to endure 260˚C for more than 10 seconds.
  • There should be no kink nor tight fit design for the pins of PIH parts. Otherwise, it is very difficult to put these parts on circuit boards. If the parts are put on circuit boards by manual operation, they may be inserted with excessive pressure. Such circumstances may cause the vibration of circuit boards, and then the mounted parts may shift or drop.
  • There should be flat surface without void on the top of PIH parts for the nozzle of SMT machine to pick up. In addition, a tape can be pasted on the surface of PIH part to avoid the leakage of air.
  • A 0.2mm gap/standoff should be reserved in the conjunction of PIH part pins and a circuit board. It is designed to avoid not only the siphon phenomenon but also the solder overflow. The issues may generate uncertain solder splashes influencing the quality of product.
  • The height of PIH part pins is recommended to be higher than PCB thickness for 0.3 ~ 1.0mm. Since most of the parts with PIH process are applied in external connectors, the pick-place operation may be influenced if the height of PIH part pins is more than 1.0mm, and the insufficient solder issue may be brought about if the height is less than 0.3mm. For smaller PIH parts, the extended height of circuit board should be smaller, too.

The impact of substituting the conventional SMD parts with PIH parts or SMD+PIH parts on process and design is summarized as follows:

  • Working Hour: There is no significant difference between the time-cost of these two kinds of parts.
  • Cost: The cost of PIH parts may be more than that of SMD parts since there are more pins for through hole.
  • SMT Part Space: According to the experience, it is better that the space between PIH parts should be 1.5mm or more. However, the space of SMD part gap needs only to be 1.0mm, even 0.5mm. The larger space is required because the pin of PIH part is easier to distort. In addition, the larger holes are also designed. The pin diameter/hole diameter rate is recommended to be 0.5-0.8 in general. Also, the mounting deviation is larger, so the larger space is required.
  • Rework: Generally, the rework and fix of PIH parts are more difficult than those of SMD parts because the solders in each hole should be removed for PIH part replacement. Although the operation of whole part break can be considered, PIH part reworks will still lead to more complex operations relatively.
  • Space Usage Rate of Circuit Board: Because there are pins in the back of PIH parts, the available space is relatively less on circuit board.
  • Solder Filling Issues: According to regulations of IPC-610, the solder filling rate of hole for part pins should be more than 75%. Some original limitations, however, may cause solder paste volume issue for regular solder paste printing. Therefore, extra solder volume may be required sometimes.

In fact, the solder joint strength of PIH process is larger than conventional SMD process, because the product can be processed by more insertions and extractions. According to the experience, about 1.5 times of strength is enhanced based on the number of pins and the size of each pin.

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