HIP (Head-in-Pillow) Defect Phenomenon: Possible Causes in SMT Reflow Process

Possible Causes of Head-in-Pillow (HIP) Phenomenon in SMT Process

The HIP (Head-in-Pillow) defect looks like a person resting their head on a pillow, not buried inside it. Recently, some people refer to it as HoP (Head-on-Pillow). Both terms refer to the non-wetting defect in the solder joints of BGA (Ball Grid Array) components. Personally, Workingbear think it looks more like a neck or waist, but I believe calling it the “Ball and Socket” or “Double Balls” Phenomenon seems more appropriate.

The primary cause of HIP defect is when the BGA components on the PCB experience warpage or deformation due to the high temperatures during the reflow process. This warpage separates the solder balls on the BGA from the solder paste on the PCB. As the PCB passes through the high-temperature reflow zone and then cools down, the deformation of the BGA substrate and PCB gradually returns to its original state (in some cases, the deformation may not fully recover). However, at this point, the temperature is already below the melting point of the solder balls and solder paste, meaning they have solidified. When the BGA substrate and PCB return to their pre-deformation shapes, the solidified solder balls and paste come into contact again, creating a solder joint shape resembling a head resting on a pillow.

Most HoP/HIP defects are caused by PCB warpage, and these defects are often concentrated at the four corners of the BGA package. This is because the diagonal distance in a square BGA package is the longest, resulting in the maximum warpage. Additionally, there are some HoP/HIP defects distributed in other locations of the BGA package, which will be discussed later in the article.

How to Detect HIP (Head-In-Pillow) Soldering Defects

Based on the description above, HIP issues often start at the edges of BGA components, especially at the four corners where there’s the longest lever arm and severe warpage. In this case, using a microscope or a fiber scope can be attempted. However, these inspection methods typically only show the outer two rows of solder balls, making it challenging to identify issues further inside. With today’s high-density circuit board designs, this observation also requires ensuring no components block the view, posing limitations.

Moreover, detecting the HIP Effect is generally difficult with 2D X-ray inspection machines since they mainly provide a top-down view and may not reveal whether BGA solder balls have necking issues. X-ray with tilting and rotating angles could make it possible to observe HIP necking phenomena. Sometimes, issues can be detected through In-Circuit Testing (ICT) and Function Verification Testing (FVT). These testing machines often use bed-of-nails fixtures, applying additional external pressure on the PCBA and may separate solder balls and paste on pads if there is non-wetting or weak solder joint. However, many defects still make it to the market, and such flawed products are quickly identified by customers due to functional issues, leading to returns. Preventing the occurrence of the HIP Effect is, therefore, a crucial challenge in SMT.

Alternatively, screening out PCBA with HIP defects can be considered through burn-in (B/I) testing (if it is board-level B/I then requires elevated temperatures). The increased temperature during B/I causes PCB deformation, giving the opportunity for open solder or solder empty or non-wetting to become apparent. However,  diagnostic tests need to run concurrently during B/I, and if the HIP location isn’t on the diagnostic’s testing circuit, the issue may go unnoticed. Generally, diagnostic testing programs for BGAs, being crucial functional components, should be executable.

Currently, reliable methods for analyzing HIP defects involve using Red Dye Penetration and Microsection Analysis, but both methods are destructive tests, so they are not recommended unless absolutely necessary.

The recent breakthrough in 3D X-Ray CT technology allows effective inspection of HIP or NWO (Non-Wet-Open) soldering defects. While becoming more prevalent, the equipment cost remains a challenge. If needed, it is recommended to consult with various labs to utilize 3D X-Ray CT.

How HIP  (Head-In-Pillow) Happens and Its Root Causes

While the HIP Effect occurs during reflow soldering, the root causes can be traced back to material and design flaws. At the PCB assembly factory, it relates to solder paste printing, component placement accuracy, and reflow oven temperature settings.

Here are several potential causes for the formation of HIP defects:

  1. BGA Package

    If a BGA package has solder balls of varying sizes, especially smaller ones, it is prone to the HIP Effect. Additionally, inadequate temperature resistance of the substrate in BGA packaging can lead to substrate warpage during reflow soldering, contributing to the HIP Effect.

    (Warpage of substrate, inconsistent bump size)

  2. Solder Paste Printing

    Uneven solder paste volume printed on different solder pads or the presence of so-called vias-in-pad for BGA pads without proper electroplating and plugged can prevent BGA solder balls from making contact with less-solder pads, causing the HIP Effect. Also, misalignment or deviation of solder paste printing from the PCB’s pads, often occurring in panelization scenarios, may result in insufficient solder to connections, leading to the HIP Effect.

    (Insufficient solder paste volume, printing misalignment)

  3. Pick & Place Machine Accuracy Inaccuracies

    Pick & Place machine accuracy Inaccuracies or improper adjustment of XY positions and rotation during component placement by the pick & place machine can cause misalignment between BGA solder balls and pads on PCB. Additionally, when placing IC components on the circuit board, the pick & place machine slightly presses down a specific Z-axis distance to ensure effective contact between BGA solder balls and the pads on PCB. Insufficient downward force on the Z-axis could prevent some solder balls from making contact with the solder paste, increasing the chances of HIP.

    (Inaccurate XY placement, insufficient placement force)

  4. Reflow Soldering Temperature (Reflow profile)

    When the reflow soldering temperature or heating rate is set inappropriately, issues such as non-melting of solder, PCB and BGA substrate warpage, and board bending may occur, leading to HIP Effect. Referencing the article on “Possible Causes of BGA Simultaneous Solder Skip and Short” to understand the analysis of BGA voiding and solder short resulting from significant differences in CTE between the BGA substrate and PCB, along with excessive TAL (Time Above Liquids) causing board warpage. Additionally, it’s crucial to be mindful of the heating rate in the preheat zone; if it’s too fast, it can cause early volatilization of flux, leading to solder powder oxidation and wetting defects. Moreover, it’s advisable not to set the Peak Temperature too high or for too long; it’s recommended to consider the temperature and time recommendations for the components.

    (inadequate reflow profile that results in component & PCB warpage, Lifting of BGA bumps due to wetting force, Excessive Peak Temperature, too much TAL)

  5. Solder Ball Oxidation

    After completion in IC packaging facilities, BGAs undergo functional testing using probes. If the probe’s cleanliness is not well maintained, contaminants may adhere to the BGA solder balls, causing poor soldering in SMT process.  Moreover, if BGAs are not stored properly in a controlled humidity and temperature environment, solder ball oxidation can occur, affecting the solder joint’s integrity.

How to Improve and Prevent HIP (Head-In-Pillow) Soldering Defects

Now that we understand the main causes of HIP come from the FR-4 of the circuit board and the deformation of the IC substrate due to high temperatures, there are two main approaches to preventing or avoiding HIP.

  • Increase the rigidity of the circuit board material and IC substrate. Using higher Tg (glass transition temperature) materials, Tg ≥ 170℃, can enhance rigidity, but it comes with increased costs. Most materials for lead-free printed circuit boards typically use medium Tg, Tg ≥ 150℃.
    Related Reading: What is Glass Transition Temperature (Tg)?

  • Increase the amount of solder paste to fill the gaps formed by the circuit board and IC substrate warping due to high temperatures. Ensure that BGA solder balls maintain contact with the solder paste on the circuit board throughout all reflow processes. However, be cautious not to increase the solder paste amount too much, as it may lead to soldering short circuits.

For more improvement methods related to HIP (Head-In-Pillow) soldering defects, you can refer to the following articles:

  • Can Increasing Solder Paste Improve BGA Soldering?
  • How to Solve the Head-In-Pillow (HIP) Voiding Issue in BGA Solder Balls

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